I can only speak from my experience within the ESSDA comps and so far I've seen all kinds of combinations:
Switching leads during the dances, switching from dance to dance, staying in the roles of leader/ follower all the time. It probably depends on the proficiency in ones skill as well as the height difference whether people switch or not.
For example, my partner is much taller than me so it is making sense for her to be the leader. Furthermore, I have no leading experience while she is quite the accomplished leader (and an even more fabulous follower) so I'd be worried to hurt her if I were leading. Another fellow couple has the same height, however the (male) follower has no experience as a leader whatsoever, so they do not switch either.
On the dressing topic: In Standard there is everything from traditional (leader: tails; follower: gown - that's what we are doing, too) to specially made nice costumes which are kind of a combination. You also see two tails; one in tails or similar while the other wearing some kind of modified suit (e.g. costume with pantskirt); two etirely different type of costumes from what one is used to (considering "regular" comps);...
In Latin pants seem to be the way to go

But you see a lot of girls wearing dresses, too.
Once I saw a gentleman who was wearing a bright orange jacket with... well, it wasn't a float, but it had an interesting touch. And another one who was wearing something similar to a tailsuit only it was a one-piece and it had a float as well as parts made of lace.
Never seen two women wearing typical ballroom gowns and - so far - never seen a guy in a skirt/gown though.