Yes you can connect in a FRST loom.

I couldnt tell you exactly what to do with your loom etc because it is a very long process removing all the redundant wiring.

But in a nut shell if you remove everything from the loom connected to the 2 ECU plugs on the Mk4 RS loom thats pretty much that, and then you can join in the fiesta loom.

A Mk4 EFI loom is a better way to do it in my opinion though.