ditto what everyone else said - even six angels in a 75 will be too much when they hit maturity and start pairing off, IMO. I've got a 90g with 6 angels in it. The smallest's body is the size of a silver dollar, and the largest is now about 3" accross (not counting tail). My two silvers have paired up (no spawn yet), and as a result, the tank seems crowded. The silvers defend half of the tank from the other angels, and there's tons of posturing and squabbling going on. It's JUST posturing, and no actual damage being done, because it's a large, heavily scaped, densely planted tank. I can't see leaving all six of them in the 90g once they form more pairs - some will be moved to the 60g I've got coming, or I may move a pair to my new 30.
That's my long way of saying don't buy more fish - it won't fix the territory problems. If you do a really good job of scaping your 75, then everyone *might* live peaceably in it, but really, you'd be best to either trade in the pair, trade in the unpaired angels, or keep the pair in the 40 and move the others to the 75.