Yeah, unless Amar improves considerably (don't hold your breath), Williams will technically be a PF, although on offense more of a combo guy who will stretch the defense. Ellison might be the guy at SF, unless Felix improves considerably (again, don't hold your breath).
I think SJU got a real steal in Ellison. He is a guy who as a Junior & senior could realistically develop into one of the go to scorers on a team that should by then have multiple 5 star players.