For size: 8" is consider large, 10" would be freakish, 12" would be a monster for a yellow tail. 18" in wild, average is 6" in an aquarium. Spiny eels will forage for food, but most often will accept worms. A feeding tip (since they are nocturnal) is to take a small plastic cup (like diners give you for salad dressing) and fill the bottom with blood worms and cover the worms in gravel, put the cup in the aquarium before the lights go out. The gravel will keep most of the fish away, and the eel should find them. They can be VERY tough to get to eat once introduced...I've heard stories of eel fasts lasting over a month before eating. I have a tire track that's about 8" - 10" and he loves earthworms (chopped). The smaller eels are considered delicate in comparison with the larger varieties (yellow tail is a smaller variety). They can be tough to get established but once they do they usually are good to go and quite hardy. They will hide, a lot. Most fish keepers think they've lost them for weeks, months, days...until they pop up...usually in a filter. They are expert escape artists so water level and tank lids are important. They hide in the substrate and/or tunnels...usually adding PVC piping creates a "home" for them...lots of spaces for them to hide will lead to them being more active, or at least visable. I've seen some gregarious eels, but most are shy. The Frontosa might consider the eel a meal if he is large, possibly thinking he's a worm! A large frontosa will easily consume a small eel. My tire track lives in a tipped over tiki head in my tank, I'll try and remember to grab some photos...always has his head out, will eat from my hand and is actually pretty tolerant of my interactions with him and the other fish. The larger ones can be hand-tamed (tire track/fire eels) but I have yet to see the smaller varieties act in this mannor. I would recommend tracking down a tire track or fire eel if you want something outgoing and not just background noise (preferably at a larger size...6-8")...beware though, they get quite large...just youtube "large tire track eel" or "large fire eel" to see what I'm talking about...they are a hoot once they hit 2 foot. They can be quite greedy!
