It will still only take the fire eel less than a year to be pushing 18-24”, so unless you plan on upgrading in the next six months, I recommend you hold off on getting another fish.
I will say that with enough water changes, you might get away with adding another fish, but we are talking around a 90% water change at least once a week, maybe twice, and then depending on the type of filter, it may need cleaned out once a week.