Groomsmen in same color but different suits?

We did this. The groomsmen are all business professionals that wear suits regularly for work so my husband asked them to wear a gray suit that they either already owned or purchase one that they would get use out of in the future. We provided them all with the same tie and pocket square. Even with the different shades of gray and different suit styles you could still tell that they were in the bridal party and it didn't look weird at all.

I think it will look totally fine to have them all wear a plain black suit! As long as they aren't trying to sneak in pinstripes or a weird lapel, they'll look super similar

FH considered telling his GM to just wear a gray suit if they have one, or rent if they don't, in order to make it less expensive for them. IMO its easier to just give them the suit rental, tell them were to go and the style. The cost of suit rentals isn't excessive and I think most guys expect that when they accept to be in the party. Some may not already have a nice suit, and I don't want them doing the leg work of finding the rental place on their own, etc.

We plan to buy their ties for them.

Why would it look strange? Women do this all the time with bridesmaid dresses, we tell them a color and let them chose a style. Same thing to me.

FH was in a wedding where the groomsmen all wore black suits they owned already. We were skeptical that it would look funny, but it looked perfectly fine! You really couldn't tell.

It worked so well, and was so easy, that's actually what we're doing. FH is buying a tuxedo, and we've told the GM to just wear black suits and white shirts. We'll probably buy them ties so that part will be more cohesive.

We are doing mix match suits and dresses.