Oh no, you shouldn't have asked me these questions, I am way too informed about them.
First off, yeah, I can totally imagine these being sidebranch of a sidebranch path later, and am at peace with them being low priority compared to finishing out the main paths first.
So now let's talk about contemporary genderslang 101:
When I use boi, I generally use it to refer to a young transman (aka born with girl parts, but identifies as male). It's not generally used that way, but only useful here as a way to distinguish between all the possible configurations of sexual organs and genders that Taylor can end up with. Think of it as an inverse of futa (which trans people consider to be a slur when used to refer to real people, btw).
A masculine flat-chested girl is still a girl. The best term we have for these are tomboys, I guess. (Confusingly, tomboys are slang for lesbians in my country, but that's another conversation).
A sissy is a very specific kind of a very feminine guy. While there are many terms for feminine gay guys, the most commonly accepted of which is femme (as opposed to all the derogatory insults available, including pansy and sissy). Sissy, in fetish terms, usually refers to an originally straight guy who is forced/pressured/brainwashed into dressing up as a girl and taking on more feminine characteristics. This usually has elements of cuckolding and being forced to participate in gay sex as well, usually as the bottom.
If Taylor gets pregnant in his female body, it is technically indirect mpreg, but Mpreg enthusiasts (OF WHICH THERE ARE DOZENS OF US! DOZENS! AND ONCE WE FIND OUT HOW TO COMMUNICATE WITH EACH OTHER NOW THAT TUMBLR HAS GONE TO SHIT YOU WILL RUE THE DAY YOU CROSSED US!!!) wouldn't consider it to be true Mpreg. They usually consider a transman (aka male presenting, but with female genitalia) getting pregnant to be mpreg, as well as a masculine hermaphrodite getting pregnant as Mpreg, but the "ideal" mpreg is a masculine dude with male genitalia only getting pregnant, and having to give birth via his butt. (Or his dick, but even most mpreg fans think dickpregnancy/dickbirth to be too fucking weird for us. Kind of like how there are furries who are too weird even for the bigger furry community?)