Rules of thumb are overly vague and generalizedl. For example, just about every fish I have is about as thick as a penny, so obviously a 1" chromis is not going to create the same bio-load as a 1" puffer fish. I'd bet that ONE of my goldfish produces 10x the waste that both of my Tangs do.
Secondly, the liverock probably only displaces around 5-10 gallons, and its all built up on the back wall, so there is tons of swimming room. 70/5 = 14". Clowns and the chromis are currently about an inch or less. The hippo is about 1.5", and the yellow tang is about 2". Cardinals are around 1.5" Thats 13.5". I'm currently under the "rule of thumb."
Third, I freakin love this hobby. Love love love. I built my own stand and I already have designs laid out for a new stand. IF, when these guys grow up (and fish don't grow fast) there is not enough room, there is about a .0005% chance that I will still have this small 75 gallon tank. I'll probably be up to a 125 by then. (More likely by the time I move next fall)
And finally, all the fish hang out near each other anyway. None display any territorial behavior whatsoever. The tangs do a lot of swimming. The chromis, clowns, and the cardinals are usually within 12" of each other. The Tangs are the only ones that actually use the space I've given them. All the others just play around in a group.
So I do appreciate the input, but I wouldn't worry too much about overcrowding. I do about 12 gallons worth of water changes each week, so the water stays nice and clean. And the fish will certainly outlive this tank. The 75 gallon size was chosen because of size limitations of my apartment. But I'm buying a house next fall, and I'll probably design an entire room around this aquarium.