Tank is cycled

[Warm-blooded]

The rule of thumb is one inch of fish per gallon.So you might be able to get 2 pair of swordtails or mollies for example plus a cleaner fish

Not true at all, unless your taking the length, width, and height of the fish all into account and multiplying it together to get the "inch per gallon" Keep in mind/the equation depending on the fish, it may grow much larger so add that in. However, disreguard this rule use common sense instead if your tank isn't wide enough for the fish, even if the gallonage comes out to being enough.