Missing parts seem to be a common occurrence in Koi Angels however, I would not have purchased a fish that was deformed ( missing fins in a genetic deformity) as you don't know what else is wrong with the fish from the start. As peaceful as you say that red lobster is, I have my doubts when it comes to slow moving fish such as Angels. Also, not sure I would put Angels in a tank with the painted tetras ( I'm assuming you mean the Glo Tetras and not Painted Glassfish) unless they were in there first. Fish combinations can change based on who goes in first and who goes in last.
Bottom line since you don't really have a good knowledge that the fish were healthy to start with plus you have some questionable tank mates in there, it's hard to say who is "at fault" for this outcome. I would however, recommend strongly that all new fish go through a quarantine period before directly adding to your main tank. IF they are in fact sick, you will be spreading those diseases to your other fish as well which is a major problem especially when dealing with stores that use centralized systems or no quarantining practices themselves. QT is a "better safe than sorry" approach to fish keeping.
Sorry I could be more definitive for you.