It's pretty straightforward to have multiple OpenERP servers running on the same machine. The trick is, as you already noticed, to have each one listening on a different port.

Using distinct listening ports

To set the port used by a server instance, start it with the option --xmlrpc-port=<port>. This can also be done in the configuration file, if you are using one (i.e. starting the server with option --config=<myconfig.conf>).

Using distinct database roles

There is no need to setup different Postgres instances ("clusters") for this. I would recommend you to use just one, and properly manage the different roles and databases inside it.

It's not required, but I find it to be a good practice to create a distinct role for each instance you'll be using. E.g:

sudo su -c "createuser --createdb --no-createrole --no-superuser openerp-test" postgres

And then, when starting the server add the option: -r openerp-test.