It's usually a matter of choice these days, so go with what you feel you;d like.
Choice of users can often come down to the number of male relatives you;d like to include in some way on the day. If your wedding is a reasonable size - say anything over 75 at the church, the four ushers is fine - just make sure they are relaxed about their duties, it's not a competition or a race, so as long as everyone gets a 'turn' at seating guests it should be fine.
I think it is good maners for the groom to pay for all the dress suits, as it is good manners for the bride to fund the trousseau and bridesmaid's outfits - but again, it depends on the choices - and budgets - of those involved.
If you and your partner can't afford to pay for outfits, there is nothing wrong at all with mentioning that to the individuals involved. If they are cloe enough to b e bridesmaids or ushers. they are close enough to kno that you;d love to pay, but can;t manage it, and they can each stand the hire themselves.
Hope this helps.