Monster hunters are a pain. I don't think they do anything until they've petitioned. Once they have, they may amble down to the caverns if they feel like it (it's apparently more interesting to read your books, pray in your temple, and socialize in your tavern: don't be surprised if it takes half a year of keeping the fortress vulnerable to the critters below before the bugger decides to haul his lazy butt downstairs, walk around for a few minutes, and then return back up because he's hungry/thirsty/tired.
Once they DO actually get down there, they cause additional danger to the fortress beyond keeping a passage open in that they spread body parts around the cavern, which causes your dorfs to rush there to pick them up. Occasionally the body parts are from some "horrible" critter killed, but often the parts are from the monster hunter itself, luring your dorfs to the location of the monster hunter's killer...
Also, any merc types, including monster hunters, tend to have crappy equipment and no armor use training whatsoever (so if they DO have armor they move at a snail's pace). Monster hunters can't be used for anything apart from littering the cavern, as you can't assign them to any duties. They probably never would petition for citizenship even in the unlikely case of surviving for two years.
Regular mercs petitioning become residents and can be used as militia squad members in all positions except leader (which requires a citizen). They never petition for citizenship. Apart from this leading to them not being usable for anything but militia, it also means they're rather likely to go nuts as they're incapable of mood balancing efforts in the form of trinket collection (requires hauling) or any type of crafting. They can read books, though.
Petitioning performers do nothing useful for two years (they've been observed to harvest crops *extremely* occasionally), after which they petition for citizenship. Once they're citizens they're on an equal footing with everyone else, so you can assign jobs to them. Some have had a useful career before taking up performing, but you have access to no information about them when they petition (Legends Viewer coupled with regular exports of Legends Mode info can provide info). Performers sort of act as migrants with a two year buffer period, so their main use would be to provide you with a multi racial fortress.
Petitioning scholars behave like performers, apart from them researching for two years before petitioning for citizenship. After that they're yours, just like performers.
Note that it's not unusual for residents to go bonkers during the two year waiting period, and you can't do anything about it as you can't control them and you can't assign them to mood enhancing tasks.
Taverns, as mentioned, are basically mandatory. Just don't appoint poisoners (a.k.a. tavern keepers and tavern performers) unless you're prepared to deal with over serving (if you want deaths to over serving you probably won't get it much, while if you don't want it you'll probably do, based on what I've seen reported).
I haven't heard anything about changes to couple beds. Once I've "encouraged" couples to marry they'll get a bedroom with a single bed.
Finally, don't send out raiding parties or your fortress will eventually die from corruption of militia equipment resulting in DF crashing. The only known way to avoid the crash after corruption has occurred is to disband all militia and never appoint any new one, with the attendant issues regarding dealing with insane visitors and camping invaders. Raiding is simply FUBAR, although it takes a while before it bites you (it took half a year or so before the cause of almost all crashes with the current version was identified).