Turns out those fry are dalmation Mollys, not guppies! They're starting to develop their "spots".
So, instead of all of these fry hiding out in plants I bought from petsmart, the more likely story is that I just was so inexperienced I didn't notice that my female Molly was pregnant when I bought her, and she birthed the fry at the same time as I was planting...
In any case, they're doing pretty well. Here's a picture of one starting to show his spots:
In other news - I ripped out all the Egeria Densa that was in this tank - I just can't keep that stuff alive. A number of the stems were disintegrating and it took a LOT of time to clean up the mess, so I just removed the rest of it.
Some of the healthier-looking ones are floating in my 29G for now, but I don't know if I'm going to keep any of it at this point.