Usually 15mm stuff is 1:100, so the HO/1:87 is a little big. Having said that, many newer 15mm infantry figures, especially once on bases, don't look bad with 1:87 stuff.
I'm not a
FOW player but use 15mm for moderns, and use a lot of HO scale civilian vehicles and had a couple HO Bradleys.
The black truck in the following picture is HO scale:
Here is the HO Bradleys followed by a line drawing to see how they should compare with a human:
And here is an actual 15mm Bradley (about 1:100):
Hope that helps a little.
Jake