Well, it's kind of difficult without any context. All we can do is say what we like and, since our tastes differ, it's not going to be very helpful in your decision. ;p Be that as it may, I'll see what I can come up with.
First: I normally do Loli tfs for these kind of things, but I was wondering if I should do something else, to 'expand my horizons'.
I wouldn't worry about this just yet. Since you've yet to do so
here, just start out by writing what you're familiar with, then decide where to go from there. Loli TFs are fine by me, anyway.
Second: Should I add some form of stereotype to the tf, and if so what?
Well, it'd be difficult
not to write something stereotypical. I think the problem is trying to avoid writing something like that.

The best way to know, however, is to write something and see what kind of reaction you get. I mean, it's more than just the stereotype, but also how it's presented and used in the story that matters, too. So, while, in one case, the stereotype would be acceptable, the same stereotype could be unacceptable in another case.
Third: should people except that they are who they are? I'm not going to do reality change, because that takes out part of the challenge of being changed.
Unless they wanted to change genders in the first place, I'd say that they shouldn't. Of course, one of the few exceptions would depend on the person's personality, and how and what they care about. That said, if the focus is how a person is changed, and how they deal with it, acceptance should be gradual, at the very least. If the focus is how it affects others, or reality itself, then it's not much of a concern.
Finely: should I add something to the change? Like catgirl or something.
Wait, isn't that a change, itself?

Well, if you're really not sure what you want the end result to be, make a poll and let others decide. Majority rule! Woo!
I hope that helps, somewhat.