I understand. I did not know this.
Unfortunately it is impossible for me to work on something that is that specific.
On the other hand, if the service offers API docs, and if you were to hire a PHP developer, I can provide guidance to them for interconnecting to the plugin.
As for your other question, there is no specific number. The plugin is built to run on sites with up to a few thousand users at most, anything over that and you might run into scalability issues, depending on your hardware, plugin settings, and services offered.
with regards