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A starter character (e.g. your first character on SARP) cannot start role-play at a rank higher than P3C (or its equivalent) without GM permission. I'm not going to give that permission, mainly because there are people in the plot who started as P3C and will soon be getting their promotions to P2C after several months of RP. Not fair to them that you would get it off the bat while they had to work for it.
Not terribly hard to get promoted. Just stick around and it'll happen.
History is good. While you give him some great background, I wouldn't say it justifies any promotion. The lack of incoming fire tends to prevent rapid promotion in rear-area units.
Since he's a combat engineer, you should emphasize what particular things he can do. A combat engineer is, after all, different from electrical or mechanical engineers. That would be the only thing I'd want to see, so I know exactly what he's capable of and what might differentiate him from the unit's other engineers.
I've tried to emphasize that he is suited for field work, and more of a hands on engineer than say a mechanic. The skills I gave him point towards a more mobile workstation kind of character. I'm just quite honestly stumped on what more you want to see. A demolitionist marine would be better suited to blow a bridge while a mechanic could repair the power armor and vehicles. I guess I tried to make him the link between a full fledged mechanic or electrical engineer and a marine. That's the impression I got from the description of combat engineers in this faction. I can see where everything is a generality, but that is because I don't know the composition of the unit. If I had to choose one thing he would be good at, it would be the maintenance and field repair of gear.
That would make him more of a mechanic or a an armorer than an engineer. Engineers are the guys who are supposed to design, construct and/or disassemble things for the other troops.
For example, you need a bridge built? Or a new base built? That's what the mechanical engineers would design and build. These are the guys who usually take on the more massive problems that aren't going to require them to be in combat.
You need a shield system set up and hardened against electronic attacks? Electrical engineers. They handle the electronics and ECM stuff.
Combat engineers handle stuff that goes on during the fighting. Got a minefield to clear? Need to fortify the hill you just took and expect that the enemy want back? Need to clear out a town ahead of the the advancing troops and hold it for the cavalry to arrive? That's the combat engineer stuff.
Sorry if I seemed clueless, but the Marine Specialty description had absolutely nothing to do with what you mentioned. I built the character off of the small paragraph that was for combat engineers and it made them seem like armorers and in-combat mechanics.