In regards to FEP permission,
zyph announced that its contents can be used as long as every creators of that pack will be credited. But if you feel that its too much hassle to merge that with Variant and probably disrupts the balance you have with the currently included classes, then not merge them. Experiment, publish, republish, nerfed down, or OPed it all...they are all your choice, no one holds you to do what you want only you can do that, we don't have policies to abide, no Runic Laws for you to be prosecuted with...you have your own copy of the game, you have access to tons of mods, and you have GUTS. Just be brave.
As a modder, I have no way to restrict my mods, anyone can learn from them and edit them for their own benefit or pleasure, if you are pleased to share them to others then do so. We credit people and ask permissions of course, but they are all just courtesy. We can have different opinions about it like
@Phanjam kindly gave to you, but all in all, I like to think that we are doing these things to share and/or inspire others. Probably that's why Steam Workshop doesn't give option for TL2 profited works/mods unlike on other games.
On these types of situation, I always based my moralities on game's creators and staffs, even the game itself. Like my Steam Tags, I followed the tagging shown by
Runic Games. For crediting and author's preferences on one's mod, I share
Blank and
Abe's principles. Lastly for my mods' standard and quality, I based that on the game, well, at least my own experience with the game. You can feel that the game was built on fun, that's why its so inviting for anyone to try and change/improve how it is.
On another note, I'm sorry that my opinion is too long, maybe see this as warning.