It really depends on what you want. I have a 10 gallon divided three way, a Betta in each section with a heater and filter. And one my Betta is in a 2.5 gallon kritter keeper with a heater and he is doing great.
flaring i believe is an act of trying to "scare" off another rival betta fish... he or she is trying to protect his/hers territory.... its also a great way for them to excercise but i do not recommend trying to have them flare 24/7... you can make them flare by putting up a mirror next to their tank so they can see their reflection... they will flare becasue they think its another betta fish... you can also make them flare by putting their tank next to another betta tank... but DO NOT PUT 2 BETTAS OF ANY KIND TOGETHER.. this will induce fighting and possibly death of one or both bettas plus its very cruel to the fish... hope this helped.. if you have any questions just ask :D
hmmmm that might be why... try moving him somewhere else and see wut happens to his eating habits.... he might also still be getting used to the new tank and everything... that always happens when i get new fishies