I have to say that I personally do not like GWs plastic Imperial Guard infantry since the proportions are rather awkward, and that being un-armoured they do stick out next to all other manufacturer's items, especially Wargames Foundry. If looking for generic troopers, IŽd suggest using Pig Iron instead, or if you insist on the GW stuff, you could use some of Pig Iron's separate head and weapon bits to break up their common appearance.
I'll yield you, though, that combining parts from the Catachan and Cadian boxes can result in some excellent figures, as exemplified by some exciting "Not-Colonial Marines" which I saw some years ago but cannot seem to find anymore.
Otherwise, IŽd suggest getting some of Copplestone Castings "Military" Future Warriors. ThereŽs a variety of soldier types in various states of equipment, and theyŽd work better with Foundry Figures (although considerably chunkier).