AFAIC, the code itself *must* reserve memory stacks for each active races once runtime routines kick in... then, the actual pool of available ID (at the settings phase) is a built-in range of available slots to deploy 12 Majors, 1 Custom, 8 Minors, etc, up until the maximum (code driven, again) of 36 in total... including the mega-events random races.
Even if you were to add any majors above the default 12 (typed 4 and filling other's reserved IDs), the code still needs to allocate memory for these extra slots which i'm 98% sure has to be done through function calls & declarations; that stuff being absent, the whole UI won't recognize any supplemental items. Considering also the extra AIs algorithms necessary to drive gameplay processing too, presuming the 9 opponents max rule still stands AFTER your list is properly detected, it would most probably take up as much memory as any single default Major slot *per* any new ones.
There might be an indirect way to "modify" the code... if you'd have access to the source;
-- Providing for a selectable screen of more choices (as indicated by your snapshot) that ties each slots with switchable incode definitions. Say, Terran 1a & Terran 1b... toggling upon one or the other.
-- Then a re-parsing routine that resets every code calls to the new race parameters **at runtime**.
-- Increasing the number of active slots for the opponents too since all races can fill such duty also.
-- Rewriting the variable arrays that keep track of *supplemental* stack space, dynamically.
If they HAD to reduce memory trace of the ships' graphics to make room for the new Gigantic map -- it tells me that the 'compatible' stack of available heaps is already somehow tight in the code.
Raising that figure, may just be too complex for the kind of feature you want in.
Besides, you could always use my X-Worlds' trick; simply switch RC.xml files that has as many different races as you wish within the default pool of twelve.
What puzzles me even further about this "need" to have more than the RC races on screen to select from is this; what stops you from using the Custom slot? It switches to another screen where you can always load up many saved "indirect" races!