Regarding the plecs, I’m not sure it's a good idea to put them in this size tank. Depending on the type, they can get to over a foot a half long. I had to return mine to my lfs recently as he got too big, and I’ve got a 260L tank (He grew to over a foot in length in just over a year). I think, plecs deserve at least a 5-6 foot plus (400L plus) tank due to their size.
As far as angels go, I have two at present and they don't bother anyone else in my tank. They fight with each other from time to time, but so far that's all.
I would also concur with the other posts, in that a betta shouldn't be put in.
There are different sized plecs. Don't bushy-nose plecs get just to be 3"-4"? Those would certainly be fine in a 55G tank. It all depends on the type he wants really.
Karl
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