So if I place a SSL for a domain on one IP then I can't host other domains under that iP? (they dont need SSL)
You could not.
Normally, an SSL certificate should be installed for the account user that owns the domain for which the certificate was generated.Any SSL installed onto the main shared IP will only be installed as the user nobody via WHM, so if you are using that main shared IP for an account, you'd first have to install as nobody in WHM, then go into root SSH and do the steps I had noted (Miraenda user is my non-staff account).
If you do not want to go through such a hassle, simply install a dedicated IP onto the account getting the SSL, then you can install the SSL using WHM onto the user's account rather than using the user nobody.