Kroot, dont sell themselves to the tau exclusively at all, the Kroot arent a single unified entity (though most races arent either the kroot are even less unified), as such thought the tau empire consider kroot to be allies, many kroot chose to find employment elsewhere, they arent even spectacularily rare as mercernaries for imperial forces who arent willing to bend the rules a tad.
Kroot are very easy to add into mount and blade, so long as you can make a decent head/face mesh, the zombie body becomes an almost perfect kroot body with a retexx, and then all you need to do is make new calves/feet. (Im not sure if the current version of m&b has the zombie .objs floating around in commonres, if it doesnt it shouldnt be hard to find someone with an older version whos willing to upload the .obj)
Kroot would actually fit quite nicely if you added in a large renegade city, cities on chaos held worlds dont discriminate and all sorts of aliens and abhumans prowl the streets looking for work, or the weak to prey upon.
Orks, you should not try unless you have a rigger and an experienced modeller, otherwise they will just look comically dumb, noone expects professional quality visuals, but green men with poorly modelled ork faces just ruin immersion, and detract from the overall quality of the mod. and quite simply there wouldnt be enough orks on a daemon world to justify the effort
also those cyber gladiators are sick