First there is the 1 inch/gallon rule, this only works for slimbodied fish (like small tetras or guppies). You should for instance be able to keep 5 female guppies (at 2 inches each) in a 11 gallon. Remember it is ADULT size that counts. Even if your CAE is only a few inches long now, he will grow up to 10 inches if properly cared for.
Then you need to research your individual fish, to get at the other rules. Some fish are:
territorial- you cannot keep 2 redtailed sharks in the same tank even if the inch/gallon rule seems to allow it, 2 male dwarf gouramis would also be a bad idea. Male bettas are likely to be aggressive not only to other male bettas but also to other fish with long flowing fins (like male guppies). Kribs can cause problems at breeding time as they want to take over a small tank and keep everybody else hemmed into a corner.
heavy waste producers- you need to allow a lot more room for a fish like a pleco, otherwise they will poison the tank with their waste
fast swimmers- it is cruel to keep danios in a 5 gallon, they need the room to move, at least 20 gallons
predatory- birchirs with small fish is not going to work (well, the birchir will be happy

), adult CAE's also cause problems
sexually aggressive- keeping livebearers in a ratio of less than 2 females per males can be very stressful, kribs are best in pairs
fin-nippers- e.g. lemon tetras or tiger barbs are not safe with longfinned tank mates
schoolers- fish that need to be kept in schools (e.g. danios, many barbs, most tetras, glass catfish, corys) will feel stressed out if this need is not satisfied; this can lead to them either getting very aggressive or timid and more prone to disease. moral: if you have not got the gallonage for a proper school, do not keep this type of fish.
heavy breeders- if you are going to keep livebearers, or a pair of kribs or convicts, it is a good idea to think through the implications of them breeding first. Livebearers (guppies, swordtails, platys, mollies) almost certainly will, convicts and kribs may. Have you got spare room for any fry, survival of the fittest is all very well, but they may not all be eaten, is there a contingency plan?
There, have I forgotten anything, folks?
Obviously, not all these factors are going to apply in your present tank, genesis, just thought I'd rattle it all off while I was at it. In your case, I would reserve that 11 gallons for either a pair of dwarf gouramis or a pair of kribs. You will need to be quite good about your water changes as dwarf gouramis in particular can be a little sensitive. If I understood you right, there may be a bigger tank coming for the rest (MTS, it happens to us all...). I would still see about getting the CAE rehomed. IMO lfs should not be selling this fish without a warning sign.
