In MI/WE, it's no guarentee when/if you will get the reinforcement, so choose the initial 16 units wisely.

The "use up all archer's arrow then bring in monks" trick in the old STW just doesn't work anymore.

The good thing though, is that, during the deployment phase, there is a "circular rotation" button (maybe you have been using it already) that will allow you to traverse through all you have available, thus you have complete control over which 16 units you want to start with.