My participation in this thread happened when the change to email addresses was forced and I could not use my own gmail ID that I had created separately. But when I tried again after a long time of not using the gmail ID, I was allowed to convert to it. With no idea why it didn't work at first or did eventually.
I've always used a non-Apple email address as my Apple ID, first my Earthlink address, then my Gmail address. To the best of my knowledge, once Apple moved from "user names" to "email addresses as user names", you've been able to use any address you chose. At this point, without seeing the error messages, it's almost impossible to say why you had a problem using your Gmail address as your Apple ID.
For even more mystery, when I logged into this conversation in a browser it prompted me for a verification code - which popped up in a nearby area of the same screen.
You've apparently set up Two-Factor Authentication.
Two-factor authentication for Apple ID - Apple Support
And now it seems like you can log individual apps into different accounts. Is that a good idea?
I'm really not sure what you mean by that. Apps that you download are always tied to the Apple ID that you used when downloading them. If you mean something different, can you try to explain in a different way?