I want to add a seperate remote sql server on each service node. Is this possible? Right now i can see database servers settings only in the management node.
No. Plesk Multi Server has a centralized management of the remote databases. When you add a remote database server at the management node, it will be added to each service node as well.
The using of default database server can be managed by the option of Service plan. You are able to add a couple of database servers and store the databases on the chosen server. By default, a new database will be created on the service node's database server.