Looking at the comparison photo the Northstar FIW are more chunky with bigger hands and overall build.
From my own collection, Eureka and Perry are entirely compatible. When I got the figures from Eureka at first I thought the Perrys had done them, they were so similar.
I have recently ordered some of the Northstar riflemen to mix in with my Perry ones for a bit more variety. They do look a bit more slender but for more dispersed rifle units they should work okay.