I know Sigrun says Finnish mages, not scouts! That's why I didn't say that she explicitly stated she had never met Finnish scouts, only that her comment suggests it. (At least, to me. I feel like she has a tendency to overstate/exaggerate her understanding of things, rather than to be precise about what she does not know.)
And, yes, as you say, her working with Finns before seems unlikely. I suppose working with Swedish cleansers (and their scouts) would be possible, since there is no language barrier, and we know from Emil that Norwegians do this from time to time, seemingly for fun.
But yes, even if the concept of 'scout' exists everywhere (maybe apart from Iceland) it's unclear how far afield they are supposed to explore. (Although, even in Sweden... I know it's not mentioned on the poster, but, logically speaking, wouldn't some amount of scouting be helpful BEFORE the Cleansing begins? To identify promising directions, dangers, etc? The Cleansers absolutely do not seem like the type of people capable of doing this for themselves. And the boundaries of what is immediately useful to explore would move every year, wouldn't they? So maybe Swedish scouts have more immediate reason to explore than Finnish ones, given the amount of cleansing they do.)
That said: based, again, on my "reading" of that is going on in the comic (in full awareness that I have no proof), the way Lalli immediately slips into a routine of 'memorize a map, go into unknown territory, observe, report your findings' suggests to me that this is something he is very used to doing.