A problem I too found annoying. However you can turn it off. If you click the grey left arrow type thing near the mini map, you will see all the tickable options. Simply turn it off. Also you can reorganise your own units, just crag the unit of the same type of the other unit of the same type you want it to go into. Say you have the 3 man price knight regiment and a 5 unit knight regiment. Simply click and drag the 5 man unit onto the Princes 3 man and you'll get an 8 man uni with the Prince at it's head. Should sort your problem.