When either swapping to different slugcats for players 2-4 on expedition OR using Change Player 1 in Expedition, only vanilla + DLC slugcats are selectable, and custom slugbase slugcats are skipped over.
Currently a workaround is possible for players 2-4 via first selecting the custom slugcats in the Story menu's slugcat select and then going back to Expedition, but that is not possible for Player 1, even with Change Player 1 in Expedition installed and enabled due to Expedition automatically swapping your slugcat to the campaign appropriate one whenever expedition is opened or an expedition campaign is selected.