Check Engine & Trans fluid questions

[jersey]

I have been getting a lot of check engine lights lately. A map sensor code showed up on and off a couple weeks ago. I added some fuel injector cleaner and that code went away and hasn't come back.

Now I got another check engine light and got two codes that read as p0128 and c0268

The c0268 reads as ABS sensor malfunction which is straight forward enough.

The other code though (p0128) reads as "Coolant temperature too low for thermostat regulating temperature"

Since when is running an engine too cool a problem?

The suggested repair is to change out the thermostat?

According to the gauge the temp is registering at the 1/4 mark, so i'm not sure what temp that is, but is there such a thing as too cool?

Outside temps here at the Jersey Shore have been fall like uppers 40's at night, mid 50's during the day.

So some advice please should I disregard the code or change out the thermostat?

With 137K miles on it, should I be changing the Trans Fluid or will doing so lead to problems? I ask, because the quick lube place I went to said they wouldn't change the fluid on any car with over 100K miles on it as a policy. That mechanic, if he even was one, said General Motors vehicles don't handle the change well, but wouldn't elaborate beyond that. So what do you think? Is there anything to that, or is it a bunch of mularchy?

Thanks