Looking at using SOVIntFloor01 as the floor for a tavern I'm designing.. but every time I load the mod my player falls right through it. Everything is lined up and snapped in properly.. is this a piece that is not usable as a "standard" floor piece?

Yes, that floor has no collision. You can add your own collision plane in the CK, or just add another floor piece from the CK and place it a little bit under the faulty floor.

...or add a collision plane ('C' button in main window).

Also, for future reference, if you hit F4 it toggles collisions as black mesh outlines. Then you can see what pieces have them, and where exactly. It may help diagnose future issues like this.

Ah-ha! Thank you! That was driving me nuuuuts!