Simple answers to complex questions.

Well, the computer probably won't notice. Although it does build up its army faster. The main thing, as Sneg said, is to capture 2 initial mines, and also the one that is important specifically for your race (for example: demons - sulfur, elves - gems). And you also need to give your hero a decent starting army, i.e., take troops from the second hero. To be honest, I've never built buildings for troops on the first turn. I should try it, although the mines can be captured with the armies of two heroes.