I have never used Pro Mail, so I cant really say if it will work with power store.
I think it would be better to ask this question of them:
Can Pro Mail connect to a third party database
really the question is whether pro mail can only use their proprietary database, or if it can use a third party database.
Does the orders table need to be formatted in a specific way? using Specific column names?
the format of the orders table in Power Store will not be able to be changed and keep power store working. so if promail needs to use different column names than what are already created, then the two wont be able to work together.