I'm looking into buying a kydex holster, but I'm wondering if the edge of it (or the sweat guard) could make it inside the the large trigger guard. Am I being overly cautious?

YES.

with a properly made holster, this should be a non issue.

I recently started making some Kydex holsters as something to do during this miserable winter we are having. I have a VP9 and S&W Shield in 9mm so those are the two guns I have been playing with initially. The holsters I have made for my VP9 don't seem to have any issues at all trigger wise. That would be the least of my concerns. What I have been fighting is the retention and the amount of retention that I get when molding the holsters. I just received my bluegun molding gun this past weekend and I was able to prep that out nicely with proper blocking and such that retention seems to be no longer an issue. I had been using my real guns to do the molding and taping them up which is  real PIA because then you have to remove all of the tape to test fit the gun etc. here is a few pics of my most recent taco holster for my VP9.