LOL! I just discovered Outpost and made an order earlier (not in your genre, though).
16.5-17mm is big? I recently got into 15mm (doing Colonial and will do zombies), and every figure I have so far averages is at or near 17mm to top of head (Stone Mountain, Old Glolry, Peter Pig, Rebel Minis).
I figured out the scale at almost 1/96, and decided on 1/100 for simplicity (and that's the scale I used for a ton of buildings I made, so I hope all the other manufacturer's 15s are similar

) NO WAY are any of the 15s I've seen 1/120 scale. That is way too small!
Glad to hear good things about Friekorp, as I am doing an order there tonight!

Sorry I can't be of much help, I don't have any experience or knowledge of the 15mm Prussina era.
I would assume GFI 15s figures would be smaller, (like their 25s compared to others)

?