Finlaggan is only slightly larger than Isle of Arran, and will become available once the new Islay vessels are in service, so I would think she might be a possibility for the Campbeltown service for 2026.
In principle anything in the CalMac fleet will fit into Campbeltown - the Ro-Ro berth has a maximum length of 120 metres and is cleared to 5 metres below chart datum, which exceeds the size of LOCH SEAFORTH. And HAMNAVOE, for that matter. In practice, the safety of the berthing operation would still need to be demonstrated before revenue service could start, though I doubt there'd be any issue.
I'd expect that the Campbeltown service will continue to be run by the second Arran ship once GLEN ROSA (and probably the four Turkish ships) enter service, rather than running a third ship out of Ardrossan or Troon.