Do alot of research, ask yourself basic questions...do you want corals and if so that should affect the kind of lights you get for your tank. Do you know what the steps are for setting up and cycling your tank? if not read up and try to get some info on how to do these processes. Consider also the additional equipment that the tank will need like a possible heater, skimmer, powerhead and ect ect. Theres alot of steps to consider and do before you can even come close to adding fish livestock. Also, whats your budget like this will also affect your options when choosing a tank. Ask and get answers to all the right questions before even considering fish... But since you have an idea on the type of fish you may possibly want i can say that mandarins take a very developed tank that tends to be a little on the large side just because they run out of food supply so easily. They usually only go for coepods and in many cases will starve from lack of food. The damsels dont seem to be a problem since its a relatively hardy fish. Finally, I wouldnt consider a butterfly unless you go for a larger tank although I have no experience with them. This is some basic advice I would offer and besides that it seems you have a lot more research to do just take your time have patience with the process and goodluck!
