Introductions on a Time Crunch

Hey all,

I have an annoying situation with my new landlords. My current cat, Onyx, is 18 months. I always intended to get him a friend, but at first he was sick and then we were in a not-so-great living situation (nothing dangerous, of course). So now we've finally moved to a fantastic one-bedroom apartment. The problem is I can't have a "visiting" animal, which means I have to pay out the full pet fee the Monday after the new cat gets there. I'm happy to follow their rules, but how can I tell in 48-72 hours if Onyx and the new cat will get along? Onyx is a sweet, friendly, playful, affectionate teenager who loves everyone he's met, but he hasn't been around other cats for a year now. Other than making sure the new cat is cat-friendly, what can I do? Thanks in advance! 

Am I wrong in thinking that you could pay a deposit for a second cat and then foster until you know if the cats do well together?  Even if you didn't keep the first cat you tried with, you would still have paid a deposit for a second animal.

No that would violate the no visiting cats rule