Hard to tell. I think you will be able to bang some of the companions without being locked into the romance for the rest of the game for sure, but no word on whether or not that will lock you out the other romances.
As for who you can do that with....uhh....Iron Bull is the only one I feel somewhat confident about guessing that for, but I'm also not 100% convinced or anything. It's hard to tell, because you can try to extrapolate things from their personality, but that doesn't mean it mirrors their approach to sex. Cass is a serious romantic, but that doesn't mean she's not up for casual sex sometimes....and Sera may be a pretty chill "in the moment" jokester, but that doesn't mean she's the type who seeks casual sex.I mean Cass as a "full romance sex only" and Sera as being able to sleep with before she's locked in....those are fair enough guesses, but they're just that: guesses.
It also depends on how it's set up. Bull I think is the only one they've definitely said likes/has casual sex That doesn't mean, however, that the way it will be written will involve that. You may be able to have sex with him early on, but it's in a context where you've defined your interest in him, and he'd be hurt by it....that's more of a break-up than an Isabela "we'll bang, okay?" 
blah blah what I mean is, we don't know. 