A marine contingent gives a warship options below invasion

Is anything smaller than a full planetary invasion force useful aboard a starship?

Yes; a few hundred marines can secure installations, support allies, conduct boarding, and provide a political tripwire.

The objection assumes any landing force must bring an entire planetary invasion system. Within Stargate's rules, transporters, rings, shuttles, and gates make smaller operations feasible. A proposed contingent of roughly a reinforced company to half battalion could handle station seizures, allied support, boarding, ship security, and other objectives short of planetary conquest. The contingent could divide its emphasis between landing operations and internal protection while retaining the ability to reinforce either mission. Deploying even a few hundred troops to an endangered ally also creates a visible political tripwire, while full invasion remains a task for specialist ships and larger forces.