If your tank is 72x18" footprint and the geos are only <2" that's going to be a very boring tank if you put only 6-8 of them in there.
I have the same tank and until this afternoon* I had the following stock:
-12" black arowana
- 8" Uaru
- 5 satanoperca
- 10 juvenile Orangeheads
- 3 adult (2m, 1f) Orangehead
- black ghost knife
- 2 pictus cats
Of course, I overfilter the crap out of it (2 FX5s and an Eheim)... But while it's slightly busy, it's not too crowded and the adult orangeheads and one pair of the juvies breed regularly.
*- this afternoon I took the 5 smallest orangeheads out and put them in my growout tank with my 6 orange Uaru. This was just because I power feed that tank and I'd like to see them grow up some. I also wanted to see how the main tank looked with fewer fish; so now I'm down to 8 orangeheads in there. But if I come across some nice severums they're getting added.
Ultimately, when the juvies all reach adult size, I'd say 6 would be the right number. But for now, 13 eartheaters works just fine.
The same could not be said of the larger eartheaters, like Abalios, Altifrons, etc, or even massive fullgrown satanoperca. But I have one 6" and 2 5" and all the rest are under 4. They don't grow that fast so you can put a ton of fish in there and it'll be a while before you have start worrying about thinning it out.