Can Professional Acrylic be mixed with Winsor & Newton Galeria?

It's possible to fully intermix Galeria Acrylic Colour with Professional Acrylic colour, either in layers or on the palette.

Can Galeria and Professional paint be mixed and layered? 

You can mix and use both Galeria and Professional acrylics together with no issues at all. You can use in layers or mixed or both in any layer quite safely.