I love Angel threads

.... so here we go:
There are two approaches to this:
(1) You should only keep one angelfish species per tank
Or
(2) Angelfish communites are ok.
We'll go on the latter

(first)
Buying juv. fish, the bigger ones are going to put on 2-3" per year, on average...... Personally, in a 6' tank, I wold not like to see anything bigger than 8-10" ..... so, in answer to your question if they'll outgrow your tank, well pomocanthus might, eventually, and the holocanthus will also. Starting out with a 2-3" fish: end of yr1 - 5-6", end of year 2 - 6-8", end of year 3 - 8-9", end of yr 4 - 9-10"......so, I reckon based on either a pomocanthus or holocanthus angel, you should get a good 3-4yrs, then you'll need to upgrade or move on the fish......
If you opted for a single angel, you'd probably push that out for another year or two....... but we are talknig communites.... so it realistically reels it in by about a year......
So - long term feasibility - I would say if you start with juv. fish, then you'll have about 3 yrs of relatively smooth running, afterwhich things are going to get a bit tight..... assuming you don; go nuts with too many tangs.
Size and aggression correlate fairly well here:
Genicanthus Bellus (f) OR Swallowtail (f) - smallest / least aggressive.
Chaetodontoplus Personifer OR Scribbled
Pomocanthus
Holocanthus
I would introduce them in two batches - getting small fish..... first lot about 3", then a month or so later the pomocanthus and holocanthus - better to get them with a slight size difference - probably get a 2-3" holocanthus and a solid 3" pomocanthus.
My personal choices - first two - doesn;t really matter, the pomocanthus - go withthe emporer, because the mac is too similar in shape to the holocanthus...... holocanthus - queen, hands down (ciliaris)......
Perosnally, if I were doing this tank, I would ask myself if I would ever be able to upgrade my tank - if not, or if in doubt, I would let out the holocanthus, and instead I would get the other three that I liked, and cincentrate my efforts on them, and maybe indulge in something nice like a hareem of flame angels......
HTH
Matt