Currently the rule reads "Populations may support up to 750 tons of shipping per Population size."

This is what I am thinking of expanding it to.

4.3.3.2.5.2 Naval Support
Populations may support up to 750 tons of shipping per Population size per month. Unused supply may be stockpiled locally (up to 10x Pop size) or transshipped to a central depot by freighters.

4.3.3.2.5.2.1 Freighters
Freighters may transport 5 points per tonnage of cargo. This translates to 750 tons or supply needing 150 tons of cargo space.

4.3.3.2.5.2.2 Supply Depots
Supply Depots would be used to stockpile supplies in forward areas, or in areas not covered by nearby populations. A Supply Depot may hold up to 20,000 tons of supply (not 5:1 cargo rate). An Empire would be able to support one Depot for each Small Core or larger population for free. Depots beyond that would cost (Curr # Depot + 1) x 5000. Each level of Manufacturing technology reduces the cost by 10%.

With this, I see armed colliers transporting supply with expeditionary fleets. Effective commerce raiding and piracy.