the truth is 10 gal tank will do for a small lmb, upto 7" bass i think.... of course the longer the tank is the better. fish will not grow big if u keep it in a small tank, some of them learn to eat pellets/flakes fast, some don't...fish happiness is just humanizing the fish, its just a fish dang it, i never saw a fish cry because his tank is too small. when u put food in the tank and it eats, than its all good...
laws and regulations, depends on the lake, some lakes says min.12", some lakes says 18", and very few says any size, but there are some.... no matter where u get them at, if u have it in your tank already, than its all good. in fact u can buy baby lmb on ebay...
i have 1 bluegill and 1 lmb that are about the same size in the same tank, both aggressive, but they don't bite each other, chase each other around the tank, and compete for food... as long as they are eating, they look happy to me...