Well, number one, it's not HER apartment. It's YOURS. And she has obviously forgotten that. She has absolutely ZERO say in anything to do with your home unless she is paying half the rent and half of everything else. And even then she needs to respect you. If she wants complete freedom to do whatever the heck she wants then she needs to move out and get her own place.
I can understand you taking care of her animals because you're an animal person and don't want to see them suffer. But you should NOT have to take care of her. And I'd tell her that if she gets that tattoo instead of paying her half of things then she can either get out or not use anything in the house that you have paid for because that is BEYOND ridiculous. Tattoos don't come before bills. I'm sorry. If she can afford a tattoo, she can pay half the rent, utilities and all of the expenses for her own pets.
Doing housework does not count as helping. She lives there too and is also helping herself by doing it. She needs to do her share of everything regardless.
Don't let her walk all over you anymore. I'd be giving her some serious ultimatums. Like...yesterday.
Oh! And no strange guys in the house! What is WRONG with this girl! What if he's a rapist or murderer or something! Then he'll know where she (and you!) live! There is a reason they say you should meet people in public for the first time. I'd tell her ABSOLUTEY NOT! And again, if she can't handle that, she can move out and get her own darn place and then do whatever she pleases!