Switching/creating new iCloud.com email?

Hello,

I currently have an Apple account (iCloud, etc) using my gmail email. However, I’m interested in creating and switching to an @iCloud.com email address to try for email use. From what I’ve heard, this will change some things on my account and I want to fully understand this before I move forward. 

If I understand correctly, when I make the switch, I’ll no longer use my gmail account for logging in but the newly created iCloud.com address. Further, am I right that once I convert from gmail to iCloud.com, I can’t ever switch back to a non-iCloud address in the future, even if I wanted to? Are there really any issues with this? Can one add a secondary email so that, so that, even if I didn’t want to continue using the iCloud.com address as my primary email, I could still get any Apple notification emails in a secondary (specifically non-icloud.com) email account? If that’s true, I think that takes out the only other “con” that I could think of if I ended up switching back bc I could still get any emails that Apple generates at another location. Am I missing any other negatives?

Are there any other things that will change if/when I switch to an iCloud.com email?

Lastly, is there any way to find out if a given iCloud.com email address is taken yet - without going ahead with choosing it (gmail offers this clearly during signup)? I’d really love to know before I hit that submit button and commit to the email choice.

Thanks so much in advance!