Well, there is not really a quick way to change reinforcements. On the strategic map, the order in which the stacks are placed in the province is also the order in which the reinforcements come one, therefore you could either reduce the number of stacks or arrange their content in an alternating manner, so that you at least get one missile unit and one unit of a different kind for every two units of reinforcements you call on. The general's stack will always be the first 16 units displayed on the tactical map.

The other way is a tedious permutation of the units at the deployment phase, there is a button with which you can cycle through all reserve units, units that get replaced by another end up at the end of the reinforcement chain. So, you can arrange the order of all reinforcements but that requires a good memory as well as patience, as the permutations needed get longer the more reinforcements you have, therefore it's not really worth it.
In short, either get VI and the patch, or improvise in MTW using what you've got.