That doesnt sound right, 4 inches of fish X 8 fish = 32... right? that many of those in a 10g sound like too much
The inch per fish per gallon is just a basic guideline, which I have never paid attention to. It doesn't take into consideration the individual bioload of each fish, the behavior of each fish, etc. A 10 inch pleco could never live in a 10 gl tank. 8 Kuhlis (while attaining 4" as an adult) in a 10 gl would be no problem. Kuhlis have a very light bioload and usually don't venture too far from the bottom "real estate" of the tank. A betta in this tank would have all the open free swimming room it could desire.
If you don't stand up for something you'll fall for anything...