Is this where "alert suppression" comes in?
Yep, you are right.
have setup an alert "node down" for the 7200's and 1800's, but should anyone or both of 7200's goes down, I don't want to recieve 800 alerts from all the 1800's
You should create one advanced alert for the 7200's that does not contain any Alert Suppresion and another one for the 1800's that has an Alert Suppression.
Machine Type is equal to 7200 somethingNode Status is down
That's how I would set this however there might be a more efficient way to do it in one advanced alert but that may become messy...