Can I
use the Twilio SMS Standard/Pro without a Twilio account?
No, however you may sign up for a Twilio account during the installation of the Cloud Action.
Can
I use existing my existing Twilio account with Twilio Standard?
Yes.
Can
I use my existing Twilio account with Twilio SMS Pro?
Most likely. If you use the opt-out feature of Pro, you must ensure that you select sending numbers that are not already used by other services.