Documentation does not indicate restrictions on user ID's, you may check "Getting profile information"
page for related details. You can choose IDs or pattern for IDs that might correlate to the old App Engine IDs. This might not prove of any advantage in the end, as in Google Identity Platform you are not supposed to use these IDs to validate user log-ins. Same applies to emails and email variants such as
joh...@gmail.com and
jo...@gmail.com. You may check the "Send the ID token to your server" on the "Authenticate with a backend server"
page.
To reply to your second question, on the same documentation page you'll notice that the get method for email profile.getEmail() does not allow for multiple email values within the same user identity profile.