In many cases, all you need hold is the port facilities, so on many smaller colonies, without sufficient terranized soil to support subsitance crops, a company or two could hold just long enough to make life rough.
Or, as is often the case in the real world, and modern Real Life begins to mirror Film, GWB is about to send a field army to do what should be don by a SO Company: remove one nutcase from power.
Inserting special ops personell can be profoundly effective, if they have sufficiently broad or sufficiently narrow engagement parameters. Need to besiege a colony? a single SO team, with the correct intell and time to do it, can shut down the fueling systems in a long-term fatal way; this makes most drops to surface one way trips until repaired or replaced.
Need to wipe out an entire colony dome? 4 to 8 SO teams can, if equipped properly, make certain that there is NO LIFE remaining.
Need a certain rabid cult erradicated, call the USDOJ's elite teams... Need it done quietly, call the SEALS or RNC's.
Alas, all special cases, but, in the 2300 setting, the Special Ops guys are hell of a lot cheaper to support, even if they take 4 times longer to train and at ten times the cost per day. And, if you do it right, they even get there on smaller, non-dedicated shipping which carries that stealth-lander.