RE: Please help me with the choice of the right Kombat volumes!
Looking at your husband first, if you already own the 96, your husband should have a pretty good feel of wether the volume is enough to float him. At an absolute standstill, the board will sink a bit because 90kg+ about 20kg gear weight is more than 96, but the more skilled you are, the more it will actually feel like a floater as soon as there is _any_ kind of movement going on. So, if there is any kind of wind left and with a bit of balance, you can get home safely on a K96 at 90 kilos. The 107 will carry 6.5 better at that weight though but I think the 96/97 will be the most all round size overall for your husband.
Looking at your, at 63 kilos you should clearly be on the smallest possible Kombat. The K79 will pretty much flat you even at a full standstill. In fact, if you have been happy on the EVO 74 when it comes to pure size, you could also look at the Pure Acid 74. It will feel a tiny bit smaller than the E74, but the sailing feeling of it is actually pretty close to the Kombats (similar rocker and bottom) and the PA74 will be easier when its windy for sure. At your weight it will handle 5.5 easily too. But you will loose a bit of stability at slow speeds, so which one of the PA74 and K79 that is the best board depends on your skill level.
Please ask again if you want more info.