The main issue is that increasing the cap requires another 3rd party script and I'm running so many already that I'm dreading a possible conflict or bugs that can emerge, but I'll definitely attempt it.
And at the moment no, all sexual acts can lead to fetish gain, however if you keep getting the same ones they will cap out at 5 and not give any more levels, so you can stock up on exp after they are capped and use it to level what ever you want and then use the leftover to sink the fetish down to zero afterwards.