They seem to me like they would both become friends because Niko kinda has the ability to see someone for who they are (Trevor was abused by his dad and then he was told he has some mental disorders by the air force's psych) and not judge them because of circumstances. Trevor would just like how Niko could just One-Man Army (like him) everyone.
I think the two would actually have some very interesting chemistry together. Both have been in the military in one form or another, so to some kind of extent they could relate quite a bit in several ways.
Now I'm wanting a story-based Niko DLC. Hopefully-maybe at some point.
Then there's always mods on PC...