Fish are tricky. Sirens have poor vision and are not particularly quick, but they will eat fish of appropriate size if they can catch them napping. Some fish will nibble the siren's gills; I had trouble with some Seminole killifish that did this. If your tank is large and well-planted, you can probably keep small inoffensive fish such as tetras, guppies, or small minnows in the tank. I often add bluefin killifish and swamp darters from the LFS' shrimp tank to my siren's setup. When he gets moved to his big new tank I intend to try orange-spotted sunfish, pirate perch, and flier with him.
Substrate is not all that important, but I would suggest that either sand or large (size of siren's head or larger) pebbles would be best. They produce quite a bit of suction when feeding and often end up inhaling a rock or two; they usually spit them out, but better safe than sorry.
They seem to be more active in cool (40-50 F) water, but can tolerate quite hot water as well. Room temp is probably a good balance.
Sirens like plants but can be rough on them, so pick sturdy or non-rooted species. Cover the tank and close any gaps. A sponge or screen over the filter intake is a good idea, especially if you have a little guy. Nightcrawlers, sinking pellets, and any standard live or frozen inverts will be eaten.