Rackham models tend to be rather tall & skinny compared to GW's, the reason being that Citadel/GW minis may theoretically be 28 mm scale, but their proportions are so cartoonishly overblown (tiny heads, large hands, MASSIVE weapons, etc.) that they're kind of a scale unto themselves.
Personally, this doesn't bother me, real people aren't all the same size ( or 'scale',if you will...) either, so I feel absolutely no compunction to not mix figures from different manufacturers.
Besides, Citadel isn't totally consistent in regards to size within themselves, the new (7th edition) Night Goblins are smaller than the previous ones, for example.
My Empire army is based on a core of 4th edition WHFB/Battlemaster plastic guys and Grenadier/Mirliton Fantasy Humans, laced with Black Tree Design Halflings and some BTD and (old) GW Dwarves.
My Dwarf army is almost entirely Grenadier/Mirliton with a few Mithril ones thrown in, those two companies make the best Dwarves, imho.