"I have already paid but I am worried that I will end up with nothing."
What do you mean you've already paid? A security deposit? First month's rent? Or are you just referring to UCLA tuition for the summer? I've never heard of, nor would I ever, paying for a room without a guarantee that the room is mine...
As for the overnight guest policy, my apartment has a similar "no overnight guests permitted for more than 3 consecutive days", but that hasn't stopped my roommate from shuttling in his girlfriend literally EVERY night for the entire year. The landlord has even seen her bags, clothes, her sleeping in my roommate's bed, her cooking...and not cleaning...and hasn't said a word. Really, most landlords I've encountered don't care about the "rules" so long as they don't hear any complaints, not that I'm complaining. You can get away with almost anything so long as nobody is pounding on the landlord's door (i.e. this IS a shot at you joy-killing, relentlessly-bickering, fun-despising, (insert lame adjective-insert childish insult-ing) people whom condescendingly toss the "I'm pre-med" line upon all of us simpletons, on the floor below me whom go straight to the landlord whenever my friends and I get a little too loud and disrupt your 8pm weekend bedtimes, rather than coming to us and letting us know we are keeping you up. Avoid THEIR wrath at all costs and you're golden. Really, as long as nobody is complaining, nobody cares. This is experience talking. You'll realize that Westwood landlord apathy is about as pervasive as used condoms on the glistening lawns lining frat row on a routine Thursday night. Alright...end rant...bullet point to take away: just bring your girlfriend over, especially since it's a single and you won't have any roommates to upset, worst case scenario, if someone complains and/or you get caught, the first time will likely result in nothing more than a less-than-stern warning, though my money would be on complete apathy.