I want to put some snow tyres on the car with different rims for the winter

Does anyone see any problems with fitting 15" rims and 195/65-15 tyres instead of the above? - the overall rolling radius is pretty much the same (within 0.5% difference)

The Focus Econetic uses 195/65-15 tyres so you should be alright (check with your insurance company that it's ok to do so).

You could fit 205/55-16 snow tyres to your existing rims, saves the cost of buying 15" rims. However, if you want the convenience of being able to change from snow tyres to summer/seasonal tyres (and vice versa )whenever you like as per changing weather conditions then having 2 sets of rims & tyres makes sense.