I have received the same answer from Check Point: "you cannot combine C500 with NGSM".
In my humble opinion, this point should be reviewed by Check Point as it takes away competitiveness versus other vendors that have more accessible and flexible on-premises management options. Check Point forces you to always go for a higher and more expensive license to have on-premises management and not to mention the cost of renewal and subscriptions (SmartEvent & Reporting and Compliance) which also increases the more gw you manage.
In the example you indicate, if you have to manage just 13 gw, you are forced to purchase a 25 gw license, so in the end you end up paying for NOT USING almost half of the capacity.