3 has been my magic number. .. and not mixing species. I would add 2 more fire eels at the same time about the same size as your current one. And add them all to the new tank together. Spiney eels will usually co-habitate well together when they are young. ( ie how you often see fires, tires, and sometimes peacocks or others in the same tank. Look, none of these fish are adults, or even sub-adults usually.) ime just as Cichlids will go stupid aggressive once they hit puberty so will spineys. Most aggression starts in the 12" range for fires and TT's and smaller for peacocks, ect.
Also to note.. My tire track is wickedly aggressive. He will take any small fish, I would honestly put my money on him verse my 3 fires which are all 1/3 larger then him. Some Fires are also notoriousely aggressive. If yours is more docile as mine is. I honestly would be very very hesitant to try even the most laid-back Tire.
No matter what you ultimately decide to do Have a seperate tank handy to split any fish up if need be.
