Hi, Mary Anne ! Thank you for your question, and welcome to Colorado.  I will answer your question about the expansive soils first.  All major population centers in Colorado are partially located on expansive soils. These soils, called bentonite, contain a clay mineral that swells when it gets wet and shrinks upon drying. Begin landscaping with a site that has been graded so water drains away from the house. Plant vegetation no closer than five feet from the foundation, unless the plants have very low water requirements. Xeriscape-type groundcovers and mulches are especially useful near houses because they can help reduce extreme moisture fluctuations. However, keep in mind that even xeric plants require water to get established, and that means for their first season in the ground.  Don’t allow sprinkler systems to spray any closer than five feet from the foundation, and plant trees no closer than fifteen feet from the foundation. It is important to carefully regulate water applied near the foundation. Avoid the temptation to remove downspout extensions to harvest water for flower beds near the house. Here is a link to a CSU Extension Plant Talk article with a little more information about expansive soils: https://planttalk.colostate.edu/topics/soils-amendments-composting/1617-expansive-soils/

Without a soil test, I can only assume that you DO have a high clay soil content in your soil there in Southeast Fort Collins, as that seems to be the norm except in areas that may have been river or creek bottoms (then you might have sandy soil) or if you are lucky enough to live in one of the "old town" areas of Fort Collins where you may have some nice compost-rich soil from years of tree leaves falling to the ground and decaying and composting naturally and adding their richness to the soil.  However, for most of us, heavy clay is the norm.  It is not all bad.  Clay, once it is moistened, holds the moisture well, and contains many nutrients that plants need.  So, there's that.  However, when it is dry, as you may have found, it is extremely difficult to dig a hole in.  So, to your question about wanting to plant your plants soon - good for you.  It sounds like you have made some good plant choices for our zone, and fall is a great time to plant.  The soil is still warm, which will allow the roots to grow a bit before winter sets in, and once these 90 degree days subside (!) the plants will be less susceptible to root shock. 

I am putting a link to a CSU Extension Plant Talk  here: https://planttalk.colostate.edu/topics/trees-shrubs-vines/1712-preparing-planting-site/

The above linked Plant Talk is about preparing the planting site for trees and also applies to your shrub (Elderberry).  You will see that it also contains links  to other more lengthy Fact Sheets which also contain great information about planting.  Yes, you should loosen the clay soil and add one part coarse organic materials (compost)  to every two parts of the native clay soil in which you are planting your shrub and perennials, ie: for every  two shovels full of clay soil that you remove from the hole and put to the side on a tarp, mix in one shovel full of compost, and mix it together before filling in the planting hole around the newly-planted shrub or perennial. Try to make sure to get plant compost, not animal manure, as animal manure-based composts can be high in salt content, and if not properly rotted can burn new plants.  You should be able to locate such plant-based compost in local garden centers or landscape materials yards in Fort Collins. Here is a link to a CSU Extension Plant Talk article about Organic Soil Amendments: https://planttalk.colostate.edu/topics/soils-amendments-composting/1604-organic-soil-amendments/

Another thing here in Colorado that is important to know is the importance of winter watering.  We tend to go through quite dry periods here in the early to mid-winter, where we may not receive much natural precipitation in the form of rain or snow for several weeks.  In that case, when it happens, it is very important to winter water trees and shrubs.  You will want to water during the warmest part of the day, when the ground is not frozen, and the temperatures are above freezing, and  finish watering in time for the water to soak in before dark.  For newly planted shrubs you should supply 5 gallons of water twice per month, when we have not had sufficient rain or snow for two weeks. These dry periods can occur here from October through March.  Since your sprinkler system, if you have one, would be turned off during this cold time, you will need to pull a hose (I like to keep a garden hose in the garage for the winter, so that the water doesn't freeze inside of it), or for a small shrub, you could just fill up a 5-gallon bucket twice per month, and pour the water around slowly over the roots, allowing the water to soak in.  Here is a link to the CSU Fact Sheet with more extensive information:   https://extension.colostate.edu/topic-areas/yard-garden/fall-and-winter-watering-7-211/

and another CSU Plant Talk article Fall and Winter Watering During Drought (which can happen frequently here): https://planttalk.colostate.edu/topics/trees-shrubs-vines/1751-fall-winter-watering-drought/

You asked about, and you will see referenced in the above linked Plant Talk article, mulch.  Yes, we do recommend using organic mulches around trees, shrubs and perennials.  Applying mulches at planting keeps soil temperatures above freezing, which protects fall transplants and allows a longer period for root growth before winter.  Around a shrub or tree, put about 3"-4" of organic mulch (bark chips, for example) starting 6" away from the trunk (or trunks, if a multi-stemmed shrub or tree) and going to the dripline, ideally.  However, if that is not possible, any size mulch ring is better than none.  We recommend keeping the mulch away from the trunk or trunks because insects, rodents and diseases can hide in the mulch undetected if placed too close to the trunks or stems.  Here is a link to another CSU Extension Plant Talk article on Organic Mulches: https://planttalk.colostate.edu/topics/soils-amendments-composting/1609-organic-mulches/

You mentioned trying to layer some topsoil on top of the clay.  Do NOT do this.  It will result in a texture interface, below which the water will not penetrate, resulting in the death of the roots of your plants.  See the discussion on Texture Interface on page 7 of the below link to CSU Extension Garden Notes on Managing Soil Tilth:  https://cmg.extension.colostate.edu/Gardennotes/213.pdf

Here is a link to a CSU Extension Fact Sheet with information on the challenges of gardening in Colorado, to help you be better prepared: https://extension.colostate.edu/topic-areas/yard-garden/colorado-gardening-challenge-to-newcomers-7-220/

If you have further questions, or if I have not answered everything, you can respond to this answer and it will be assigned back to me.  You can also always send an email to <personal data hidden>.  This email address is monitored by the master gardener on duty for a given week, who can respond to your emailed questions, or find someone who can.  Best of luck in your gardening ! 

You are so very welcome, Mary Anne, and thank you for your kind words.  I am happy that the information has helped you.  Helping people with their plant questions is  what we love to do as master gardeners. :-) Take good care, and best of luck with your new plants !