This is the first time that I have brewed Mangrove Jack's Belgian Ale and it has been in the fermenter for the past 9 days and has a stable SG reading. Is it advisable to use finings on this brew?
If necessary, yes. However you are not at the point where I would even consider finings. It is by no means necessary to use them unless you are unhappy with some aspect of the clarity of the beer. You can achieve a good result by placing your beer in a cold spot for a week or two to allow the proteins, tannis, and yeast to fall out on their own thus avoiding the need for other fining agents.