OK, so in my experience, which is fairly limited to cold water diving. 8 - 16 degerees Centigrade most peoples feet are slightly buoyant when using booties and open heal fins. Best thing is to get used to it. It usually isn't to much of a problem when you are actually diving.
If I dive in my 7mm, with booties and fins I still end up with my feet heading for the surface when I'm in the pool in the position you described.
As for the the TUSA fins. I sell a lot of them and there is definately nothing wrong with them. If you are struggling with fit then that is a different story. Perhaps get into the pool with the fins on and ask your instructor to check em out.
If the fit is right but the discomfort is still felt try some other fins. The following are examples of fins made to create less strain on your calves, ankles and feet. (TUSA Xpert Zoom, ScubaPro Twin Jets, ScubaPro Twin Speeds, Mares Volo.) I have the twin speeds but they are also quite buoyant. The Xpert Zooms and the Twinjets are less buoyant. I'm not much of a fan of the Volo's although lots of people really like them.
My wife also has buoyant feet and she uses ankle weights with a half kilogram on each ankle.
Good Luck and have fun.