Incidentally, if you are going to include multiclass kits in this function, there is no real need to glue QDMulti into it - you could of course also simply implement your own solution to achieve the same results - specifically, the creation of kit menu tables, and the use of .eff files or something similar to apply multiclass kit abilities via the single-class unkitted CLAB tables.
I suggest this simply because that would really make ADD_KIT_EX a true one-stop-shop for installing kits of every stripe. There need be no conflict; even within the some mod, some kits could be installed with ADD_KIT_EX and other with ADD_KIT + fl#add_kit_ee + QDMulti. The two methods could coexist perfectly well.
But, another suggestion, echoing one I already made to Wisp: if you do incorporate multiclass kits into this, I would put a check somewhere relatively early in the code looking at 1) is ToBEx present; and 2) is the kit single-class or multiclass... and if the answer to 1) is no, then only allow the kit installation to proceed if the answer to 2) is "multiclass." (As discussed elsewhere, the EE engine is limited to 255 single-class kits, but multiclass kits added via a QDMulti-like fashion can exceed that limit.)