Yes, it is possible mix different media-based building kits on a layout. In fact, it can add a lot of visual interest...
Hornby model railways are OO Scale or 1/76th that is 4 millimetres to the foot (12 inches). It runs on a track with a gauge of 16.5mm (this is the gap between the inside edges of the track). This is one of the most popular scales to model with...