Are the ecu's the same type ?

The mk5 ecu is the older ford style. The St150 had what looks like a Mazda ecu. If yours is the old ford style ecu then you should just need the focus ecu.
"Should" that's not a guarantee. I normally like to do the conversion and play with it as little before singing its merits.

The ecu controls the AC other than the engine but that's about it.

How the engine ecu will get on talking to the Gems, and and dash you will have to see.

I think it will work.

There are mappers that say a 1.8 will produce nearly the same output as a 2.0. It's all in the mapping to make sure there is no det or massive boost spikes.

I do it differently . I build the engine to match the mapping as its cheap and quickly and plug and play.

If you can get your focus ecu to work then the focus Rs ecu will work. Drop a 2.0 in and away you go.