These could indeed be fungus gnat larvae. I found a couple of sources that indicate that fungus gnats will eat roots and invade the stems of affected plants.  Seed corn maggots start with seeds in which the maggot destroys the germ of the seed.  Both insects love damp cool soils with lots of organic matter in them. With the corn seed maggot the seed is generally destroyed causing there to be no emergence of a plant or a very weak plant.  Since you started with purchased plants ready for planting I tend to think that your issue was most likely with the fungus gnat.  The maggots your photographed could be either critter as the descriptions are very similar, I suggest that you take your plants along with your larvae to your county Extension office for a definitive identification. Perhaps they can put them under the microscope and determine which critter it is.  The Linn County Extension Office is located in Tangent, at 33630 McFarland Road.  Call for directions and to ensure that a MG is on duty.  Phone is<personal data hidden>.