I'd like to think that some of the characters had existing relationships prior to retreating to Finland. I'm not necessarily thinking deep personal ties, but at least a collegial professional relationship. Perhaps a long-standing correspondence or that sort of thing. I'd be perfectly willing to have Galan have some kind of connection with one (or more) of the other magi. It seems to me that having some kind of personal connection like that would be the basis for the basic level of trust. HE could certainly be one of the magi that Calpurnia corresponded with.
Why go to FInland? Well, because it looks like a safe place to go. There are few enough safe places to go in Mythic Europe these days. THe thought of losing the Gift should be terrifying to a magus. That kind of threat would make the unthinkable thinkable. Galan doesn't have the benefit of being from Finland the way Angarr is. But he's willing to go anywhere so long as it has the potential to be safer than central Mythic Europe.
Is Angarr right to be suspicious of the magi's motivations? Well, yes and no. Yes, the only reason that they probably came to Finland was because of the plague. But some of them may be going all in, trying to make the best of a bad situation and getting invested in the local area. I think it'll be a good dynamic for Angarr to work out how the various magi feel about the move.