Rossin Columbus SLX f+f, or not..?

So we have made out that it is definitely a Rossin but just with unoriginal forks. Would frame builders ever use different
branded dropouts of the front than the back or would they always be matching?

They can be different.

Easier to buy them both the same, as bulk from same manufacturer reduces costs. But there's no reason at all that they have to be the same, and you order fronts and rears rather than whole sets, so perfectly normal for them not to be the same.