I keet reading that rule of thumb for a fish only tank was 1 inch per gallon,
rule of thumb you have there....
is 1" of Adult sized fish, per 5 gallons of water, which allows for 6" of Adult sized fish. I stress adult, because those fish will grow, so you need to know what the average adult size of the fish will be. The rule of thumb is really a guideline, with the right selection I would say you could go 1" per 3g. HTH