Hey there,...not sure if you have tried yet, but I highly recommend that you DO NOT put your eel in with the puffer.
I found myself in a similar situation when I picked up a 13" Fire Eel in Grand Forks on a wknd visit.
My puffer had been in a tank full of feeder guppies when I first got him and he never touched one. I added a couple of small plecos to help with the algae. No problem.
Added my Fire Eel (also named Elvis!?! weird.)....and he hid in one corner, the puffer stayed in the other. No problem.
Turned out the lights,....PROBLEM. Fire Eels come out at nite and they like will try to curl up and cuddle with any fish they can't eat !!
He was all over the puffer, curling up on his head, swimming around him, beside him, etc.
I came out the next morning and Elvis had two large 'chomps' about 4" from his tail. One right thru his body, the other caught the bottom edge of the tail.
I transferred the eel to a hospital tank, then to his new home with Bichirs and ropefish once he had healed.
The puffer is now living with 8 or 10 guppies and the two plecos. The plcos hide really well and the guppies are too small for him to bother with, I think.
If you care about the eel, I would suggest other arrangements. The puffer will not necessarily try to eat the eel, but I can almost guarantee that he will kill it sooner or later. He might tolerate it for a while but it's not worth the risk IMO.
HTH
Blaine