but sending emails from a @pm.me email address requires a paid account.

Forgot that, I personally have a paid account and I think many people using Protonmail do, the free accounts are not so good.

You should never use an e-mail address from which you cannot send.

Depends for what, if you subscribe to newsletters you don't have to send emails, but I agree it's pretty dangerous to use an email that you don't have send access for.