I know that the lenses for the Fives Squared and Fives 3.0 are compatible between the models. Are there any other frames that use the same lenses as another frame?

Splice and Unknown do, as with the Flak Jacket and Half Jacket 2.0. I believe the M2 lens can sort of fit certain M Frames, but it's not perfect. Original Half Jacket lenses can sort of fit into a Romeo 2, but again it's a matter of fiddling with them, and it's not exactt.