from my personal view...18" thickness...with 80 long...personally...5-6 TOPS full grown....id like to see what others would have to say too...
saw your edit.....i have 4 caribe and 2 redbellys...i love them and wouldnt change a thing about them...the caribe are makin my reds not so skittish...but my reds are somewhat enforcing the tank so...i love it....go with reds, caribe, ternz, or piraya's...or get one of each
depends on your filtration id say 6 is the key # but some under stock them which is probally great for the fish usually the 2 extremes are harry with the low and if hater chimes in i think he does well with the overstocked tanks so maybe he will give you the max high # but those two are great for giving the wide range then you can pick a # between them depending on your filtration
I would go with 8. As stated above, if you stick with reds you can definitely get away with overcrowing the tank. However, a mixed pygo shoal is very sweet. I would start off with at least 10 reds since you will lose some along the way.