GZG's "Alien Mercenaries" Line already forms the core of my (currently small) 40K Ork force. I'm hoping to expand this with some suitably "looted" human vehicles from various lines that produce trucks and post-apocalyptic vehicles. I'll also be converting some 15mm goblins to act as Grots, but mostly as banner bearers and non-combatants rather than fighting Grots.
I'm also creating a reasonable approximation of Tau by using GZG New Israeli bodies with Critical Mass Games' Ygs Merc heads. Just by sprinkling a couple of these converted mini's amongst unconverted New Israeli's I'm hoping to give them a much more Tau feel. I'm also looking into various 15mm grav vehicle lines that I can use to represent Tau vehicles as well as certain power-suit lines for Tau Battlesuits.
I've got my first batch of Tau-headed infantry on the painting desk as we speak. Should have pictures up by tomorrow at the latest.
For Imperial Guard Rebel Mini's Earth Force troopers make damn good Cadians, but they suffer from a distinct lack of officer figures.
Khurasan's Neo-Soviet line make for pretty damn good Old School Rogue-Trader style Imperial Guard. The Infantry squad's come with a pistol armed leader who can easily be armed with a chainsword. The RPG-esque guys work as Grenade Launcher troops and three packs of infantry squads will also net you a heavy weapons squad of three missile launchers.
Then there's the heavy weapon pack that will provide snipers, mortar teams and lascannon/autocannon teams and a High Command pack that will give you an officer, a commissar, a medic and a guy standing at attention who is just begging to be converted into a platoon standard bearer.
For rogue Trader games, I'll be ditching the weapons teams, using one of the RPG guys as the squad grenade launcher and treating the missile tube firer as a lascannon armed guardsman.
As for Imperial Guard vehicles? There are dozens of very nice wheeled APC's out there in 15mm scale that look very similar to the Chimaera. I might not even bother doing any conversion work on these models at all. Various drop-ships and gun-ships out there can stand in easily enough for Valkyries and the like. Moreover, 15mm WWII lines produce plenty of vehicle mounted artillery and anti-aircraft guns that make fine proxies for other Imperial Guard vehicles.