Am I being overly cautious or is my concerns real?
Your concerns are legitimate since many novice drysuit divers complain of floaty feet. FWIW, getting rid of positively buoyant fins is not always the "fix" for drysuit floaty feet.
And, yes, you're being overly cautious...but that's not such a bad thing when it comes to learning how to operate a drysuit.
As long as your weighting is correct and the distribution of that weight promotes static horizontal trim, you're good to go. I normally use Turtle fins, which are negatively buoyant. There have been dives during which I've swapped fins with someone else for a positively buoyant pair. I adjusted. No big deal. I've also worn positively buoyant fins with ankle weights. I adjusted. No big deal.
With experience, you'll learn that you can move weight around on your rig and alter body positioning to make many suboptimal/new-to-you gear configurations "work."
Just make sure that the foot pockets on your fins fit your drysuit boots. If your current fins can accommodate your drysuit boots, then you
don't need another set of fins. I had to purchase a new set of fins when I bought my drysuit, since the Bare Trek boots I have are very bulky in the forefoot. I didn't need a dive shop to tell me that.
Enjoy your new drysuit...