Can I purchase Personal Sites for a Commercial Licensed Multiple Site Manager?
No. Licensing for the Multiple Site Manager (MSM) and additional Sites must match the license of the ExpressionEngine installation. If your ExpressionEngine installation is a Commercial License, then the Multiple Site Manager and additional Sites must all be Commercial licenses. If your ExpressionEngine installation is a Personal license, then the Multiple Site Manager and additional Sites must all be Personal licenses.
Of course, you are free to use a Commercial licensed Site for personal and non-profit use, but you are not allowed to run commercial/for-profit websites from a Personal License.
