I created a blank Summoner and pushed the level up to 4.
I see the Feat "Extra Summons" in read:
Extra Summons Type: General Can be taken more than once Stacks Requirements: all of ( at least 1 Summon Monster ~ Summoner Special Ability ) and var: classlevel("Summoner")/5 at least Feat_Extra_Summons_Count Description: Source: Paizo Inc. - Ultimate Magic, p.150
In the book, it says "prereq: ability to cast Summon Monster I as a spell-like ability, summoner 1st"
So even a Summoner L1 should be able to take this, and a Summoner 7 twice.
Looking in the PCG file there is no "Summon Monster ~ Summoner Special Ability" any more.
Pathfinder core rulebooks (set)
minor issue that the Feat does not go Red when you have already selected max stacking (e.e. 2x Extra Summons at L14), but it does correctly not add the 3rd Extra Summons
If it's a big deal, I can allow it to turn red.
50/50 - some complain if it turns red, some complain if it doesn't. I coded it to act like an Archetype in the fact, once you take an archetype, it's no longer allowed by it's own categorizations.
End goal - does it enforce the rules as intended? If yes, job accomplished.
in terms of usability - if an item can be selected only once, and does not turn red, then it's unambiguous nevertheless, as it's "once only"
if an item stacks, and you reach the stacking limit (>1) but don't get feedback, it is less perfect. The user can't be sure if they are clicking the right thing and nothing happened, or the wrong thing, and if so what.
I'd vote Red.
Other than that, it does what the book says.