The great thing about OPR is it's very easy to house rule. Creating a house rule that wizards can't cast after rushing could work. Being able to move 12" then blast a unit with magic is very powerful so putting a cap on it could be fun!
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i think 12" range for magic casting in not enough. I make my pitch battles without a single piece of terrain, and even then, 12" is not enough because you need to come so close, you risk being charge by a standard infantry unit.
Rush not allowing casting spell, is a good rule. It make sense that you either run or spend some time for magic incantation. It create a good choice to make during game, whether to move further or cast a spell after advance. Otherwise always Rush is a no brainer.