Generally speaking fire eels seem to get bigger though, and thus likely will grow faster as well.
For keeping them together some people say spiny eels will fight too much, some people have a dozen in a tank. I'd lean towards saying if you have a big enough tank with enough hiding places / territories they'd be fine.
Can keep both if i got rhe tank space. Just gotta keep lots of caves and spaces to hide. Can be aggressive to their own kind. Fire eel will grow faster b
Hey I've got a dozen spinys. And I can tell you that from what I can gather from my experince is that aggreasing usually only happens between same species.
For example, my black spotted eel never bothered anyone. Then I got a 2nd black spotted eel and my original one would attack it on sight. Got rid of the new bse and the original is chill.
I have two small zebrinus eels. One is dominant between the two and will attak often although not as much as the black spotted did
But he leaves all other eels alone.
I've got 2 fire eels(13" and 16"), a black spotted eel(14"), and a tire track eel(18") in the same tank and here is what I've noticed majority of the nipping is between the fire eels but sometimes they do nip the tire track or black spot. That being said it doesn't happen very often. As far as which grows faster it really depends on what you can get them to eat at first the fire eels were growing fastest then the tire track eel started eating tilapia and jumped from second smallest to biggest in 2 months