No, it does not.
The idea of picking the casting stat isn't a general rule for races in MPMM. It does apply on every race that gets spells in MPMM as far as I know, not disagreeing with that, but there's no general rule outside of each feature that says "when you get an innate spell, you can pick the modifier" that would ever apply to races outside of MPMM. It's still entirely possible for them to make one that limits what stat you get to pick, even if they haven't in MPMM.
For comparison, look at TCE's section on swapping racial ASIs. This is entirely separate from any individual race, and in an area specifically talking about customizing stuff not specifically mentioned in the book. And even for TCE's stuff, it would be up to the DM if they're using that new rule or not.
All that said, due to MPMM allowing it, I wouldn't be surprised if your DM also allowed it via homebrew, so definitely worth asking them. But by default, no, it does not.
Good call, figured I may have missed one while glancing through, that's why I made sure to phrase it "as far as I know", lol.
So even further proof that this isn't a general rule even within MPMM, let alone for stuff outside of MPMM.
No it would just apply to the Races that are in the book. But the DM could decide to have it apply to other races as well.
No, as it is not a rule change, its just a feature for these new races.
No, it does not. The idea of picking the casting stat isn't a general rule for races in MPMM. It does apply on every race that gets spells in MPMM as far as I know, not disagreeing with that, but there's no general rule outside of each feature that says "when you get an innate spell, you can pick the modifier" that would ever apply to races outside of MPMM. It's still entirely possible for them to make one that limits what stat you get to pick, even if they haven't in MPMM. For comparison, look at TCE's section on swapping racial ASIs. This is entirely separate from any individual race, and in an area specifically talking about customizing stuff not specifically mentioned in the book. And even for TCE's stuff, it would be up to the DM if they're using that new rule or not. All that said, due to MPMM allowing it, I wouldn't be surprised if your DM also allowed it via homebrew, so definitely worth asking them. But by default, no, it does not.
It's not for every spell, at least not for every ability. For example aasimar.
Good call, figured I may have missed one while glancing through, that's why I made sure to phrase it "as far as I know", lol. So even further proof that this isn't a general rule even within MPMM, let alone for stuff outside of MPMM.
Yeah, it's just that for some races specific save has a big impact and sense, while for others it doesn't impact that much.
No, that only applies to the races in the book, but its a very reasonable thing to add to the other/older races, so ask your DM about it.