One of the challenges in mixing figures from different manufacturers is the fact that scales can vary slightly from company to company - even when the advertised scale is reportedly the same. The Miniatures Page has a good discussion of the various scales online. Flames of War uses a 15mm scale, which is roughly 1/107 assuming the miniatures are measured "eye height" as opposed to "head height." Many of their vehicles are advertised as 1/100th scale, which would be a 16.1mm scale miniature. To get around some of these issues, some manufacturers will use a "heroic" scale, so while a miniature may be advertised as 15mm, the figures are at that scale quite literally larger than life and actually measure out to a larger scale.
For my Polish Black Brigade project, I needed to use miniatures from a variety of manufacturers to actually complete the force. Most of the infantry is True North with a few Forged in Battle figures thrown in at this point. As I expand the force, I'll add a higher ratio of Forged in Battle to some of the platoons. The 75mm gun crew is a Divisional support choice, and is standard Polish Army, so those figures are Battlefront.
As you can see from the photo above, the Battlefront and True North figures match each other in scale quite well. The Forged in Battle may be a shade smaller, but the figures in the photo are in a partial crouch position.
Overall, the figures seem like they should work well together. I'm waiting on a few more anti-tank gun crews to come in before I start painting as I want to paint the infantry en masse. Given the color schemes are simpler than the German schemes (and infantry tools) I've been working on recently I'm hoping they paint up a bit quicker.