At least Gene put some thought into Starfleet's rank structure. In the original BSG, for example, Glen Larson didn't know anything about military ranks, so he arranged them in alphabetical order!
(i.e. Captain, Colonel, then Commander)
That being said, I know there's precious little canon evidence for this (other than Colonel West in ST VI), but I'm firmly of the mind that Starfleet also has Marines, with the appropriate ranks.
Edit: Colonel West isn't the only one we know of. I believe the ground team accompanying Kirk and crew to Nimbus III in ST V were also supposed to be Starfleet Marines. And Gene even wanted to have a platoon of them stationed on the Enterprise in TOS!

That being said, I know there's precious little canon evidence for this (other than Colonel West in ST VI), but I'm firmly of the mind that Starfleet also has Marines, with the appropriate ranks.
Edit: Colonel West isn't the only one we know of. I believe the ground team accompanying Kirk and crew to Nimbus III in ST V were also supposed to be Starfleet Marines. And Gene even wanted to have a platoon of them stationed on the Enterprise in TOS!
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