Additions to Florida State P&S Contract

Quick question as online info has left me confused.  For the Florida investors, with or without a license...

Is it legal to make changes to the Farbar Purchase and Sales Contract. Eg. Adding additional terms to the addendum. I want to add clauses to protect buyers side (me).

Or for eg.  On closing date , going with "...closing on or about May15th 2018" 

Vs

Closing on May 15th 2018.

Thanks again and if you have a follow up question please ask and I hope explained my questions well.  Good luck!