Re: how much rust is too much for inside your gas tank?
I have used hydrochloric acid based toilet bowl cleaner for this exact issue- but it is a lot stronger than you think- 1/4 strength or less would probably work. It will etch the tank. After the hydrochloric acid, a wash with a phosphoric acid metal prep will prevent flash rust and help prevent re-rusting overall.
Different approaches- I'm sure Pablo's will work, I just know that I have done it this way.