Hmm... I don't know if there's a great way to implement this with the core rules, but...
What if the mouse caster were to make some sort of construct, and use magic to convince foes that the construct was the actual caster? The construct could easily carry the mouse around with him, eliminating a lot of the movement problems, and shield the mouse from a lot of attacks as well. It may be difficult to work out, but would be very fun to play.
Another point about mice: unless I am mistaken, mice don't have any sort of opposable digit, which (according to Savage Species) will limit them a lot in what they can do with items.
What if the mouse caster were to make some sort of construct, and use magic to convince foes that the construct was the actual caster? The construct could easily carry the mouse around with him, eliminating a lot of the movement problems, and shield the mouse from a lot of attacks as well. It may be difficult to work out, but would be very fun to play.
Another point about mice: unless I am mistaken, mice don't have any sort of opposable digit, which (according to Savage Species) will limit them a lot in what they can do with items.