New tankmate for my Gar

Ive been looking at getting more fish in my 120g tank for a couple years. The only problem is what most of you have a hard time with. Finding a fish larger enough to live with your current large fish.
What I have: 15" Gar (Spotted or Florida, Pretty sure it's a Spotted)
10" Ornate Bichir
I visit the 2 local pet store couple times a month, and there's nothing ever large enough besides Oscars that would work with my fish. I really want another gar, but all I find is 6" or less.
My visit to the pet store today was different. I actually found a Datnoid, and fairly good size too. It's about five inches. They said they have had him for 6 months, survived their store relocation, and have become kind've they're pet store pet, but since I know the owner, he said he'd let me take for $40. They said he's been growing faster than any datnoid in the past, and he eats like a pig. I put money down on it, so I can get one of my tanks ready and pick it up later. Plan on putting it in a 55g until he's big enough to put in with the Gar. My question is, how big should he be until he would be less prone to be attacked by my Gar?

Well, my gar is a snapper ( predatorial towards anything it seems). I afraid of him latching on to the poor datnoid and possibly killing him. The gar has been mainly by himself so long that I don't know if he'll not attack anything, hell, he'll bite me or the net. I'm debating on getting rid of him (but had him for 7yrs), and buying the two 5-6" florida gar that are in the tank with the Dat at the store. That way they can grow together, and be more used to tankmates by the time they reach a larger size.
Say, I thought they were florida gar at the store, but they named them something different. Like uepadblablabla or something. Know of any pet store weird gar names? Can't quite remember the name, but want to make sure they aren't long nose gar. Is there an easy way to tell the type when they are small.

Well, I picked up the Dat today, and instead of putting either gar or dat in separate tanks, I just put the Datnoid in with the gar and kept a close eye on them. The datnoid swam right up to the gar, and actually pushed him around a little. The are actually getting along perfect, which I find hard to believe with my gar. Heres a pic of my Dat, not the best pic, but alright. He is 5" on the dot. I tried to get a pic with the gar, but the dats just too damn fast.