you'll have to go into the script and edit it!
to disable the custom quiz, open up the script "unionall", Ctrl+F 'coro EVENT_M00A_01', and replace that entire coro with the following (you can also just delete the unnecessary stuff)
here's what it'd look like before:
and after it'd look like this:
now, to keep the custom quiz but have your own custom starters, you'll want to navigate to the system scene S02P01A, and under acting, find "newtest3" and "newtest4". open their scripts...specifically, newtest3 is where you'll change your hero options, while newtest4 is for partner options. choose a Pokemon you want to replace, and replace all mentions of it in the script with your chosen Pokemon. also replace all instances of its index number being called upon. so say you want to replace female Glameow, look for index number 1073 and replace it with the one from your desired Pokemon! (this may be trickier with a Pokemon with a smaller index number, for male Bulbasaur for example you only want to replace the "1"s in special processes that are actually meant to be its index number; you can figure it out by looking at the other Pokemon)
sorry if that's a lot to go through lol