All those races are implemented and usable as RPCs, why would it be necessary to disable the base ones?
It may have been planned to integrate the 5 races as choice for players,
the string database contains description for them along with those you get on r-click for the classic Wiz8 races,
the existing limit is probably buried in the exe, like many other things,
and the editor does not offer the option to add an additional choice on character creation.
However there is a possible workaround with a savegame editor,
the editors capability to change a characters "race byte" is probably not enough to trigger an adjustment of races modifiers and preconditions in game
but the savegame editor also allows to export a character from any saved game to the roster,
this character comes with all race specific modfiers, you can you use him with adjusted attribs/skills/profession for a new game.
Confirmed to work for saxx, myles, madras, sparkle, sexus, vi, and rfs incl his "no magic" flag, they also keep their portraits,
and you can either assign one of the standard voice sets or replace one with the RPCs specific set.
That way we could create a set of new PCs which are based on the games RPCs,
no matter whether as standalone package or as part of a mod which are based on the games RPCs.
btw you can create a character specific "spcl_item" for those premade PCs (or any other)
it has some restrictions that provide a spcl behaviour in game, technically spoken it prevents the transfer to other PCs:
Create an item with desired modifiers which is equipable, equip it with the PC in game and save, in case export the character.
Edit the item usage type from "item" to "none" in CF item editor.
The item will keep its benefits/penalties for the character as long as it is equped, once unequipped it cant be equipped again
With a minimum of interpretaion of the guardia scenario,
it would imo make sense to have any of the other races availble,
the Higardi/Helazoid as a space traveling race, its not unthinkable they had specialists from their home planet on board their ships.
Or even some blind passengers in the cargo bay, from their last visit on lv-426