Grimm…
So this is a little theory of mine, all those so far with other grimms in stories they either hate all wesen or kind of insane, to me at least. So, my theory is that with the background of working with the royal families in the past, the grimms are not wesen haters inside, they are hunters. I imagine that they just came across as wesen haters after all the bad ones did and the fact they have or want to beat them otherwise wesen will try and kill them and relatives.
Think about it, it is a terrible burden to carry a little as those that grow up know they will get their powers if a relative dies. Wesen are born with them underneath, but grimms are passed down after death.
Therefore, they are basically hunters. So I had a thought if another grimm came by Nick's place, and somehow gathered all his wesen friends in one place that because Nick's grimm knows that these are 'allies' or 'friends' to it, it might protect the wesen even though there is a fellow grimm in front of him. Also, they might even have their own instinctual language like in Charmed where whitelighters clack like dolphins.
I imagine that they can also woge, but they don't really do it. So when the other grimm comes into his territory, Nick's grimm can tolerate it like the wesen but when it threatens its friends, it comes out. Nick's grimm immediately comes out and places himself in front of his friends. The wesen behind him are surprised because they thought it only happened to wesen, while the grimm in front of Nick is disgusted by doing this in front of wesen.
So Nick's grimm is all, 'these are mine, all mine and not you prey' or something and later it turns out okay, cause the grimm chased the 'inferior' grimm away.
Nick's grimm talks with the wesen in his life for the first time face to face but it generates to him going to them and saying 'mine' because he is really possessive of his friends which he tells the wesen that they are his friends.