When small you could probably have them all, but once they start getting larger and putting on some girth you may run into problems. You could always upgrade or sell a couple if/when the need arises. I dont know what your eel experience is, but you said you have 2 snowflakes? What I did was 1 large feeding once a week then do my waterchange the day after, which works well the once the eels get large and eat less often and this approach may enable you to be highly overstocked once they get larger.
As for other eels, depending on availablility, some other cool eels Ive seen include:
yellow margin (Gymnothorax flavimarginatus) and chestnut (Gymnothorax castaneus), both "girthy" like the green, but max out at about 3-4ft compared to 8-10ft.
chainlink (Echidna catenata) which is similar to the snowflake, the banded (Gymnothorax rueppelliae) which is similar to the fimbriated, and the reticulated (Muraena retifera) which is similar to the dragon.
You can google the latin names for pictures of each.
If you decide to go with 6 eels, I would pick the gold tail and jeweled personally, for no reason other than I like how they look better.
hope that helps a little.
Jon