What Nashoba said.
IC, Nepleslia has no reason to 'play nice' with them, especially given the risks, and the Mishhu themselves aren't known to willingly put themselves in a position of weakness. Even from the Mishhuvurthyar's point of view, they probably have nothing to gain, especially because they have holdings of their own off-map. At their very 'nicest', the Mishhu would shoot at anyone else's ships so that they can be left alone. Like some sullen, angrily smoldering Geth. Even a single one that isn't attacking people and is working with them would likely be thinking about attacking people - and in what ways it could do it - at every single waking moment. Overall, I have to agree with Fred as well.
Having Mishhuvurthar that are amicable in the setting is a really, really big deal. By having something that so sharply contrasts with established lore really does diminish them. This is especially bad for the Mishhu because they've had some consistency problems in the past, where they came off more as large-hams than sentient-monsters in certain instances.