It's actually okay to have rfts on 1 axle and std tyres on the other axle.. many people do that and find that it's okay. I know a few forum members who do that too LOL.
What is not advised is to mix the type of trye on the same axle... biiiiig no no!
As people have already suggested, sell off the part worn RFTs and put that towards std tyres.. there is quite a market for RFTs out there so if you can, go down that route, otherwise it seems diff tyres provided they're the same per axle is fine.
Now: E92 335D
Gone But Not Forgotten: E90 320D Alpine White