I'm a thick as they make kinda guy. I hate cold feet. I always dive 7mm boots (sometimes with 3mm socks). Floaty fins may make it a bit more of a challenge in shallow pools with students doing skills. IF this is an issue for you do what I do - thin boots for the pool and as thick as they make'em for ow. I like twin jets and orignial jetfins - negative.
since then this is exactly what I do, wearing 3mm soft sole boots for pool and the 6.5m fourth elements for open water, as you said, doing DSD's in 1m of water is more like floating on the surface wearing the thick boots..
James.