A light grey suit isn't really formal, but if we're talking about an outdoor wedding during the summer, expectations for formality are going to be lower generally and your guests are going to probably dress in a way that is cognizant of the heat. If you truly want to be formal, don't wear light grey. Considering the context, however, I think you're fine. Re: SuitSupply, beware the working buttonholes. It is great that they have them if you do not need to shorten the sleeves too much, but there's a reason why a lot of places get the sleeve length right and they sew on buttonholes. I think the flower options should be fine.