Holy moley! That's a lot of cats! I'll bet your garage is about as vermin free as any place on the planet!
I don't have a lot of cat experience, so I have difficulty telling the difference between male and female in young kittens, too. There is a video at the top of this page that I found helpful:
https://www.kittenlady.org/sex
As for Kitty-to-be-named-later, if you can catch her, a vet could tell you if she's expecting. I'm not sure how you'd tell from a distance. And although Bob has been fixed, there are probably other toms wandering around that she might have met.
Do worms cause rounded tummies in kittens? Maybe the belly bulge is from a case of worms? If you can get a stool sample (I know, with 15 that might be difficult) you could get a quick answer to that question....
Good luck with Kitty-to-be-named-later!