When you change the difficulty level, the avatar's rank changes. I like this a lot, but how low the rank is really struck hard. In practical terms a Lieutenant and Captain is right for commanding 30-50 people, but we're talking about a completely new international military, with the potential to grow into multiple bases in a very short time, and even the starting base can grow very fast. Another issue is, if the top man is ranked so low, he won't be able to properly command respect with existing militaries, and politicians. Modern militaries have generals buying uniforms, the whole mindset is that your ability to think is proportional to your rank. As wrong as that might be.
If this were real, the goal would be to establish the officers with the expectation of expansion, and with parity of respect. That means the top officer needs to be at least a Brigadier General. It would result in a very officer heavy organization, but only at first.
If the organization were actually a subset of another military, then the lowest rank of the top officer could be Colonel.
Another aspect just occurred to me, the rank goes up with the difficulty, but shouldn't the rank go down with difficulty? It may seem a bit counter intuitive, but presumably in the harder difficulty, you have less of everything, an everything is harder. One way to spin that is, everything is harder because you, in-universe, has less experience, less political pull, less support in general. As an example, being higher ranked in lower difficulty could explain countries hanging with the Xenonauts longer, against poorly handled invasion activity.