Since more than six months

Solution 1:

It's not advisable to use 'since' like you have done, though it may still be correct.

You can use

I have been assisting Mr. XYZ for more than six months.

Or if you want to make a point that you have been assisting someone for a specific period of time, you can use

I have been assisting Mr. XYZ since January.

Assume January is six months before the current month.

Solution 2:

Your "rule" is wrong. 'Since' is used to indicate a point in time past from which, not a period during which. 'For' is used to indicate a period.

I've only been married to Sally since July 1st.

It seems as though I've been married to her for æons.

The phrase 'as per' should be avoided. It strikes people as pompous and stilted. Say instead, "according to [the rules of] English grammar."

Don't capitalize 'project manager' unless it's an official title. (Speakers of British English may think differently about this, though).