True there was still fighters in the air at the beginning of Reach, but ground airfields were the first things hit. Restricting all air support to those provided by the fleet, which is generally limited. We have to remember that the UNSC had contingency plans to move all industry facilities on Earth and Mars to other locations if the planets were ever discovered. After Reach they began the evacuation of Mars as per that plan, and the fleet contingent that was protecting Mars, which is in fact the Home fleet and not a separate fleet, was pulled back to Earth. At the same time of the assault on Earth that covenant also hit Mars in what became the Battle of Mare Erythraeum, which resulted in a massive UNSC defeat. So logically by the time we arrive Mars should be under Covenant control. We could expect up to 90% casualties on just the orbital drop, with our team being the few survivors.

Now that I got that out of the way my suggestion is don't worry. I repeat the mantra of my last RP community, leave the details in the background. This should be a character driven RP. Who cares what air support there is, or even the name of our unit and the specs of the ship. Its just background noise and we as the characters are the focus, not the junk details.