1) Would you keep an eel in a reef tank?
A ribbon eel or snowflake, more than likely. I have a 14" Engineer goby that serves as my eel
2) What kind would you keep?
Moray is by far the easiest to care for. They have minimal maintenance needs but that fierce reputation is well earned. Beyond the fact that they are totally intimidating there are more than 100 eels in the family Muraenidae that are able to be kept in the home aquarium environment.
3) What would you keep with it?
Fish that cannot and never will fit in its mouth. They are compatible with Triggerfish, Pufferfish, and Larger Angelfish. For larger morays, maintaining water quality suitable for invertebrates becomes much more of a challenge. Realistically, you'll want to consider these specimens for a fish-only system and species or mixed large predator set-ups will be the best route.
A word of caution and feeding:
When keeping moray eels must be very careful because all species have those needle-like teeth which can cause severe damage and some victims have lost fingers, thumbs and sometimes more as a result. Though we all get tempted at times, avoid hand feeding any of the moray family. They can move exceedingly quick and with their poor eyesight often bites will occur because they can't readily distinguish between food and your fingers. So we always advise using a feeding stick, or tongs when feeding and finally you must always keep yourself alert to what's going on.
Feeding:
Feed twice a week. Also if your Moray Eel doesn't eat do not despair.. It's not unusual for new marine eels to go on a hunger strike for several weeks after introduction to new quarters, so it may be necessary to train the moray onto captive diets. Regular careful feedings using the protection of feeding tongs will produce a trust between your moray and you. It's rare for them to starve to death, so keep plugging away until they feed!
Moray diets vary. They're all carnivorous, but some require a crustacean-based diet while others will favor fish or squid. The greatest clue to any species diet is dentition. Crustacean feeders tend to have flattened teeth for grinding and crunching hard-shelled invertebrates, while the needle-like teeth of piscivorous species are used for spearing and gripping slippery prey. Whatever the diet of your chosen species, variety is important. Specialist crustacean feeders should have a range of prawns and crabs. Piscivores will benefit from a mix of frozen fish species as well as squid. Vitamin supplementation should be considered for all morays to prevent dietary deficiencies.