The NMX have spies, intelligence and double agents. Before long, it's more than likely that the Gartagens will end up being in a situation similar to the Lorath - and that turned out pretty badly. Maybe we're talking about a couple of months here, but it's bound to happen - and that sooner than the Gartagens would probably be to receive them.
Also, the mishhuvurthyar have scouted around. They know about the Iroma. They simply haven't done anything with them since, for the moment, they're uninvolved. That's probably the priviledge the Gartagens also enjoyed until they met with Yuki Toshiro and the Nepleslians.
If the NMX follow Melisson's pattern, we're looking at a NMX coalition investigating and making sure which side the Gartagens want to be on. That alone might be strong arguments for 'newer' races to remain uninvolved with the NMX's enemies, since Yamatai and Nepleslia have rather unfavorable warfronts and the Mishhu have a lot more ressources at their disposal.
Heck, the Mishhu might even want to enlist the Gartagens with encouragements to end up on the winning side and use them as battle thralls. If the Gartagens refuse, they can always be conquered, and then forced into being battle thralls.