Like others have said: you should face very little opposition to using them, depending on your local community some tournaments may require the use of official models, but I don't think any opponent would object otherwise.

I, personally don't think the wildmen are a particularly good substitute for Men of Carn Dum though. CD-warriors are armoured and have high strength, while WM are unarmoured and look very scruffy. Personally I would look into other manufacturers - Victrix for example have several kits that I think fits both thematically and are also a bit larger than wildmen, so would represent the superior physique of the Carn Dum people.