Douglas Costa plays LW while being left-footed. Griezmann was mainly used as a Left winger for Real Sociedad.
Don't know about Rashford, too early to make a decision about best position imo, although he seems great at sniffing out chances in the box.
I'd note that Griezmann has improved since moving centrally and that we don't really know yet how he would do if given extend time on the right.
Costa is doing well on the left, it's true. I think he's the type of tiny, quick player who could maybe play almost or as effectively on his same side wing. Someone like Navas was effective doing so at Sevilla. If you're not going to be a real threat to cut in and score, then sure.
But there's a reason Neymar is on the left, Messi on the right, Robben on the right and Ribery was on the left before injuries got him, Ronaldo and Bale the same and Hazard and Sanchez have become left-wingers. It just seems to work better.
I basically agree with your 2nd paragraph, though. Way too early to say about Rashford (and probably Martial too. Only Memphis looks fixed at LW). But I'd rank RW as likely to be his 3rd best position behind 9 and LW in some order, based on what I've seen, his physical profile (not short and his pace seems relatively better over long distances rather than over short ones, unlike someone like Pereira who is quick over 3-5 yards but not over 15-20) and trends in football.
We need players to rotate anyways. Martial, Memphis and Rashford covering 2 spots (LW and 9) would be more than enough games for all 3, considering we'll be back in Europe next year (if only on Thursdays).
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