There are several issues to consider before adding another cat to your household:
Increasing the number of cats confined together can really increase the chances for personality conflicts. When I moved with my cats from a 1000 sq ft apartment into a 750 sq ft one, they started having problems - the two younger cats ended up ganging up & picking on the older one, who responded to the stress by urinating on my bed (not
fun to cope with - for me or for little Felixia!). Now that we're in a house and they have more space, they get along fine. If you choose to add a cat & things don't work out, what would you do? Would you be able to move to a larger space, or return the cat, or find another good home for it?
If you add another cat, you'll need to add another litterbox too - do you have space for an additional litterbox?
Cats can live up to 20 or more years - can you commit to keeping and caring for all of your cats through that entire time? Another cat means additional physical and financial responsibility, and veterinary care for an ill or senior cat can get very expensive.
Then there's the multiple-cat mystery factor - if someone starts having urinary troubles, diarrhea, or even vomiting, unless you catch the cat in the act it can be hard to see who's having the trouble, which may mean a trip to the vet for testing for all the cats.
Regarding allergies: Each different cat produces slightly different dander (a substance in the cat's saliva that dries on the fur & floats around in the air causing allergies in susceptible people). I'm allergic and have become used to my own cats, but exposure to a new cat really bothers my allergies. An additional cat may make your roommate's allergies worse.
When you allow the cats out into the rest of the house when no one is home, they'll still spread dander about the house. It's probably a good idea for you to use allergy wipes
on the cats before you let them out into the house to cut down on the dander they spread, especially if you choose to add another cat to the mix. If your house or apartment has central air, be sure to change the filter regularly.
I'm not trying to discourage multiple-cat ownership - just trying to point out some of the issues to consider in making your decision. Whenever you take on a pet, you're taking on a big responsibility - best to consider carefully