Why a fire-support gunboat should not be disguised as a sloop

Would the proposed sloops need longer-ranged six-inch guns for fire support?

No; four-inch dual-purpose guns suit sloop duties, while six-inch fire support should come from a purpose-built gunboat.

Four-inch guns have less range than six-inch weapons, but the Royal Navy did not normally use sloops as shore-fire-support gunboats. In escort and anti-submarine service, dual-purpose four-inch guns could engage aircraft and nearby surface threats without distorting the ship's main role. A station that required six-inch firepower should receive a purpose-built gunboat, following the precedent of British six-inch gunboats on the China Station. Treaty rules did not require every function to be forced into the sloop category. Role specialisation preserves an effective escort instead of weakening it to imitate another type of warship.