you could put a pretty good amount of other fish in there if they are on the smaller side. As everyone says here, 1 inche of fish for every 5 gallons. Now, the snowflake doesnt swim around too much so he will do fine in a 55 gallon tank and doesnt need a 120 gallon tank like the formula says. Yet tangs and other open water swimming fish need a tank over 100 gallons when they are only a few inches big (dont believe me? ask how many tanks you could put in a 29 gallon tank and see how many member of the Tang Police will yell at you). The only thing is with the eel, they are pure carnivors, that means they have a lot of waste, so the trouble is more with the bioload than with the space in a tank your size. Just make sure whatever fish you introduce in to there, you do it slowly if he is realy big. It would also depend on how many other agressive fish or carnivours you put in. Like it was said earlier, just because you have a snowflake eel, doesnt mean you need to have an agressive tank. To cut down on bio-load you can add in a bunch of live rock (which will help with breaking down waste so you can add more fish, but takes up space so you cant. Its a big catch-22. But you should add live rock in anyways because the eels need to hide and its benificial too). You can also get the bacterial necesary for biological breakdown freeze-dried. Its in a little container and called bio-zyme. I have some and it seems to work okay. Live sand can also do this, but is is more expensive, i still recomend it though. Other tank mates can also help with this, cleaner shrimp have been known to pick parasites off of all types of fish, including eels and even inside their mouths. In many cases, there is a mutual agreement. These shrimp will also pick at the bottom of the substrate and help to deal with over feeding. Id check you eel out to see how agressive he is though before putting something that cost 15$ in your tank that could easily be lunch.
one more thing, while looking at what fish to add, i wouldnt go with the chart from the link given below. It is a good guide-line though, but it says eels go with only a few things things, and for the most part that is true, with the exception of a few types of eels like snowflakes because of the reasons in my last respons.
Id ask others for opinions first, ask people what have worked with their eel and see what fish you like most. But it can depend on the actualy eel you get, and not jsut the type sometimes.
let me know how it goes, i have a new tank and the guy at my LFS said my eel will be fine in the 29 gallon, when i found out differently i read all i could about him. Thats probably the best advice i have... read-read-read...Everything else you can disregaurd if you want, or check, and i encourage you to do so.