Upward Mobility?

[Nathan Ross]

As I understand it, the end of a career progression for a Roman soldier rising from the ranks, should he prove both capable and lucky, would be Praefectus Castrorum - after which he would retire from the army. I've also read that membership of the Equestrian order was a possibility for former camp prefects - (was this purely on the basis of wealth amassed, or was it a reward of service?). Would there be any possibility of continuing a career from this point, however? As an equestrian, would a former Praefectus Castrorum be eligible to serve as an equestrian tribune, commanding auxiliaries or legion vexillations (or would this be something of a backwards step, career-wise?)<br>
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In the province of Egypt, where the administration was in the hands of equestrians anyway, might a former praefectus castrorum, or even a former senior centurion who'd made the wealth qualification for the equestrian order, be advanced to command of a legion? (i.e filling the role of the senatorial legate, which I believe was also called Praefectus in Egypt), or were legion commanders in this province always selected from a pool of likely (and well connected) candidates in Rome, just as with the senatorial legates elsewhere? <p></p><i></i>