I personally believe the 1inch per 5 gallon rule is not an accurate way to gauge at all. The way it ought to work in my opinion---is add one fish at a time, then wait a good long time. (I say 1-2 months but then I tend to be cautious there) If your system handles him great. If he plays nice with the others great. If there is no fighting over homes (hiding spots) great. THEN add another. And repeat process until you notice any signs of aggression or your system is not adequately handling the load. Test often after each addition. If you find your needing to clean your tank more than once a month--then your done. Other factors in amount of fish you can add is the amount and type of filtration. Also volume is less important than surface area. Take a big tang for example, most agree that a 55 is too small for one but would say hed be fine in a 90. Well some 90's are no LONGER than a 55, just simply taller, which really is irrelevant.
All fish should be getting enough to eat without having to fight, all fish should have their own homes without having to fight, and all fish are entitled to good water quality. ASk yourself if you are willing to maintain an overstocked tank, before overstocking. Understocking is an easy way to ensure lgihter maintainance, and healthier , happier fish.
Now to answer your actual question--adding more than one or two at a time is always a bad idea--always. This package you mention--what else is in it besides the flame?? Your yellow will most likely kick his butt. However I am also unclear as to what tank your planning on adding him to. You mentioned 2 different tanks--one established and one new. A flame should always go into a well established and very stable tank. Perhaps you could throw that yellow in the newer tank, and let the flame go into your older one.
Good Luck
