i agree, do not attempt mandarins until the tank has aged quite a bit. they are very difficult to get eating frozen food, dont believe your lfs
. If they swear they are eating frozen make sure. But I still would saty clear of them for at least a year so if they decide they dont want to eat at least there will be some pods
, hopefully in abundance, in your tank. Also agree on the butterfly, another difficult fish to get eating and they will also eat pods
so they will compete with the mandarin.
And finally, you dont want two angels in the tank, even with a 6 foot 180g tank. they will find each other and fight.
There are sooo many fish you can get for this tank though, so dont be bummed that your first selection isnt perfect.
have you considered a yellow tang or a bristle tooth tang? Pretty easy to keep and beautiful fish. I have four tangs in my 180g, a yellow, blue hippo, tomini and a white lined (looks like a mustard tang lol). I also have like 10-14 chromis, a rabbitfish, pair of clowns, mated pair of filefish ( I dont recommend, mine eat coral), some misc blennies and two hawk fish. I am pretty much at fully stocked so you can see that you have plenty of room.
I am hoping to add even more chromis so I can have an uber school of them. lol But keep in mind I am meticulous with my tanks, I do very large water changes to keep parameters in check. If you want alot of fish you'll have to commit to the same regimen.
Good luck! Revise your list a bit and post, would love to see where youre at.