you can buy a 10 gallon aquarium for less than $20. House them there temporarily.

If you care about the safety of your pets, then you don't put 2 aggressive species together. I know piranhas are lazy eaters (I've had piranhas) but they do occasionally nip at stuff or if they're hungry they will investigate.

Besides, how do you plan to prevent a feeding frenzy when you feed the gator? As soon as he takes the food in the water the piranhas will come - they won't kill him, but certainly nip and possibly take out a few toes. Not worth the risk if you care about them.

Mr. F
-----
Real time Rogue, a captive Palp