RE: kombat 107
Last years Kombat 106 had a 32cm fins specced, so up to that depth is definitely OK, and the box should be plenty strong so I guess it should handle a bit more.
The original 30cm fin for the K107 is probably a little bit too small for 7.5 for most people. But you never know until you tried it, so give the original fin a go first.
As for planing limit in knots - I dare not say. Its very, very individual.