anglers are much better off in a species only tank, for a number of reasons - primarily feeding (imo) issues, and obviously compatibility - not many eels will ignore an easily caught meal like an anglerfish. There are obviously some larger varieties out there - but they are expensive and hard to come by.
to answer your other question - no, you do not have to have a sump, but water quality and environment are always critical to success with marine fish - esp. the sensitive varities, and I would think a live rock based filtration system (which usually includes a skimmer) would be your best bet (aesthetically as well).
hth, any other fish you are thinking of? plenty of good ones out there.
