Additional Free French ships would strengthen several Allied groups

How might French ships from Mers-el-Kebir have been used if they had joined the Royal Navy and Free French forces?

They probably would have reinforced the Mediterranean Fleet and then been distributed in smaller groups wherever they could ease Allied commitments.

The ships would probably have continued in roles for which they were already suited rather than forming an isolated prestige fleet. Their first effect would have been to strengthen Allied naval power in the Mediterranean. Individual ships or small groups could then move around the wider force as operational needs changed. That extra capacity would make it easier for the Royal Navy to cover more commitments without supplying every hull itself.