you should get them all at once. i dont think it matter whats you get for a while. i think eventually the gt will kill most of them once he puts some size on.
but seriously, go for the 75. i had my red devil/midas in a 55 for a while and i was very happy when i put him in a 75. the extra width really gives them a lot more wiggle room. but now, at his adult size, i feel the 4' tank just enough. its likely ill be moving in about 2 years. when that time comes, ill be moving him into a 125.
ill say it again, big fish need big tanks.
guess everyone else is extremely inexperienced or a kid.