I've always assumed that because this was the MACOs' first time in such deep space, with previous human vessels that they could have hopped upon only going so far, that the MACO brass just wanted a high ranking officer to represent the service in important matters. Having a high ranking officer like Hayes onboard gives the MACO troopers some representation in the chain of command, if not official, given that a MACO Major is close in rank to a Starfleet commander. (if comparing traditional Army vs Navy ranks). Command probably also wants to make sure that their troopers are treated right, and having a high ranking officer to advocate for them sounds like a good way to do that.
And there couldn't have been more than what, 3 squads worth of MACOs onboard Enterprise? While it would make sense that there is a senior Sergeant somewhere...I wouldn't say one is required. I can easily see a squad being lead by a Corporal, although there were a few sergeants mentioned by name, if I remember correctly. This particular group of MACOs could be organized like a special forces unit, where in each squad of X number of soldiers, the most experienced member of the team is in command, not the highest ranking. Afterall, I doubt Earth would send just regular ground pounders to fight for it's survival. Sending the MACO equivalent of the SEALs, or the SAS, or the GSG9 would make sense.
And there couldn't have been more than what, 3 squads worth of MACOs onboard Enterprise? While it would make sense that there is a senior Sergeant somewhere...I wouldn't say one is required. I can easily see a squad being lead by a Corporal, although there were a few sergeants mentioned by name, if I remember correctly. This particular group of MACOs could be organized like a special forces unit, where in each squad of X number of soldiers, the most experienced member of the team is in command, not the highest ranking. Afterall, I doubt Earth would send just regular ground pounders to fight for it's survival. Sending the MACO equivalent of the SEALs, or the SAS, or the GSG9 would make sense.