Apparently, to distinguish between male and female, on males the streamers on the fins are longer than the females, is this right?
thats correct, the males finage is longer, but both the males and females have fin extensions so it can still be tricky to sex them.
Will a bunch of males get terrotorial and fight each other?
i'm not really sure on that as i've never sexed them at purchase (also had a few i never sexed at all

). i would assume they wouldn't be overly aggressive though as they typically school when kept together (somewhat like a family unit). very hard to say though what whould occur with full adult males in a tank and no mate to spawn with. when i kept them i had both males and females so can't really give you a first hand experience with that unfortunately.
I read the minimum tank size is 20gal, would this be for just 1 or can i have many?
they work best kept as groups, so you'd be better off trying to go for a 30gal if you can stretch it. minimum tank recommendations vary a lot. heaps would disagree with this, but personally i'd keep around 3-5 in 30gal.
If i use crushed coral or fine gravel will it get sucked up by the vacuum?
i actually recommended fine gravel rather then sand (which is typical for tangs) because it's abit easier to manage with vacuuming and filter set-ups. the crushed coral can be bought from LFS in different sizes like gravel is sold (fairly large peices can also be used).
My tap water ph is neutral, can they get used to that?
How do I raise it and keep it steady if i need to? I have alkaline powder but that runs out quickly, i think there's a rock that keeps the level up high?
it's best to have your water around 7.5 - 8.0 PH, things like the crushed coral can help to raise PH as well as chemical boosting if needed. there is also a range of tang cichlid salts available from LFS's that assist in getting your tank water setup for tangs.
Can i feed the small cichlid pellets that i feed my Oscar or is there a specific food for africans?
there's specific foods pellets for african cichlids in general (as there wild diet differs alot in protein etc.). there's also specialized pellets specifically for tangs that can be used, but a staple african pellet will do the job providing you assess the ingredients.
Will they eat bloodworms/brine shrimp?
yes.
What sort of filtration do i need? (i can get fluval canister filters here)
just whatever filtration you would use on an american tank should be fine. a canister would be good if you can afford one (otherwise HOB's etc.).
How much of a water change each week?
due to their size they are not big eaters so they are not really large waste producers like alot of american or large malawi cichlids. small water changes weekly of say 20% would be good. it's important that the water changes do not vary too much in temp or PH parameters though, as tangs are a little more sensitive to swings in water range.
phew, needed a coffee break there stebo

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