I agree with 1fish2fish. It can be done, but shouldn't be done except by experienced betta keepers. From everything I've heard in a really large tank they don't need to be from the same batch of fry, but they have to be keep with clear dividers for a really long time so they establish territories, and if they flare at eachother it's time to move to a new pair. 75 gallons is big enough to house multiple males (since their territories wouldn't take the whole tank and in theory shouldn't go into each other territories) but it's not a good idea.
You could put a lot of females in there and go with a asian black water biotope (not exactly a betta biotope but it's the closest thats affordible IMO) and you'd have a great looking tank.
There are some wild bettas that you could put multiple males and females in there but you'd really need to do your research since they require a bit more than a common betta.