I got a 90 for that purpose and it really boils down to two species. The golden dwarf and the chesnut eel (not to be mistaken for a viper eel which get larger and are very aggressive). Those guys would be perfectly happy in a tank that size for their entire life. Maybe a ribbon eel, but that's an extremely big maybe. Any of those would need to be a mated pair. Best bet is to settle on the gdm and just one. (really not that rare, just ask the lfs to order you one, might take a month or two).
Anything approaching 2ft is too large, it really is. I know people list things like snowflakes and goldentail (or more commonly known is the bananna color variant) but once you set the tank up, you'll realize that even though eels don't swim much, that tank is way too small. Once I had my rockwork set up with hiding spaces and everything, I realized the same thing.
As for eels in a reef, its perfectly fine, however certain species and individuals will eat shrimp, snails, crabs, and other cuc, so that can sometimes make maintaining a reef difficult. That, and eels can be very messy for their size, which most are enormous in terms of fish to tank ratio. That being said, you can try it. A gdm would be perfect in a 75 reef, barring any cuc attacks, which will really depends on the individual. Chesnuts are fairly uncommon and can sometimes be misidentified as baby viper morays, which makes them a gamble. They are typically a little more aggressive I've read, and they do get slightly larger than the gdm (or maybe slightly smaller, I'm a little unsure now that I think about it). They both stay around 12".
I'm turning my dragon tank into a macro tank. You could always start to think about that.
If you want to get a larger tank, the options really start to open up in terms of combing eels, but nothing is a sure bet and eels, although easy to keep, need a setup specialized to their needs.
edit: The article that was linked is great, I remember reading it when I started out, however, the idead that a snwoflake can be housed as an adult in anything less than a 6ft tank, let alone a 40g is just cruel. Do the math, the full eel would be almost longer than the whole aquarium.