Who should pay for groomsmen waistcoats?

The general rule is that if it's something you're requiring them to wear, you should pay for it.

With the white shirt, maybe for your piece of mind have a back up on hand for this guy if he's a similar size to FH? And an iron in the guys' getting ready area haha

That's pretty much what I thought, but FH really strongly disagrees and tbh they're his groomsmen so I don't know how much I should interfere...

Definitely going to leave an iron in there! He's not remotely close to FH size-wise though, so not sure if I can just find a way to see this shirt beforehand and check it's suitable? He's single, lives with a housemate (a guy) and smokes, so him turning up in a yellow, stained shirt is also possible - and there's no one really in his life to check on him or point him in the right direction :/

Hmm I guess if FH thinks it's no issue then it will be his awkward conversation to have if he's wrong!

Why don't you arrange to get all the groomsmen together a few weeks before the wedding to check all the outfits? Leave yourself enough time to get replacements if needed. I think if they're wearing a lot of their own stuff it's fair to say something like - "Let's make sure all the outfits are working together before the day. Come over for a couple of drinks and bring your outfit to try on."

Kind of my thoughts - I know one of his groomsmen (the single smoker) very well, but not the other, so I definitely think leaving it to FH to sort is my best bet.
Getting everyone together is a GREAT idea, I will definitely do that and get it in my calendar as a last check.

I really don't mind getting a decent white shirt sorted if he's not got one (although at 32 years old I don't see how it's not possible, personally), I just need to KNOW.

You are correct - you and your OH should pay for the waistcoats since you are requiring them to wear a specific, fixed-price item.