To mix tiles go to Map at the top
Map Textures...
Then click the modify button there
That will take you to the data editor
In the data editor scroll down in the right pane until you get to the area that says Terrain Textures, there will be 2 boxes with terrain types in them
Texture Library is the available textures and Texture Set is the ones you are currently using
You can only have 8 at any 1 time, so remove the ones you arent using and add the ones you want from the texture library with the add and remove buttons that are between the to terrain type lists
Placing units works just like it does in WE with the unit layer
The first unit you go to place wont show up until after you click to place it