Paul's Motivation in beginning?

No offense buddy, but it would be best if you just watched the movie again, and try to stay awake this time:)

Paul was a homeless hustler. He gets picked up by some rich kid who brings him to his comfortable apartment. He finds an address book full of names of other rich people whom he can also likely hustle. That's his motivation. His reasons clearly grow in depth throughout the course of the film.

As far as Trent, it seems pretty clear that Trent is lonely and closeted. All he wants is acceptance and companionship. When Trent spoke, he was not so much hatching a plan as he was spouting wishful thinking. He's aware of how far-fetched it is, but nonetheless goes there in his desperation. In essence, Trent is simply offering Paul a gift in exchange for his company. Paul, however, took up the plan, ran with it, and made it real.

As far as Trent "assuming" he's gay, gay guys know how to spot other gay guys. There's a word for it. Perhaps you've heard of gaydar.

"Sacred cows make the best hamburger."

Paul's motivation was to ingratiate himself to these wealthy people by finding out as much as could about them so he could worm his way into their lives and eventually rip them off...Paul was a con man, in the simplest of terms, and that's what it was about for him, initially. That's how it started out, until he actual seems to develop a real affection for Louisa and Flan.