Hi Vikassingh,
I don't have a WatchGuard unit in front of me.
Before I get an eval unit, I wanted to make sure that is was possible and from the sound of it, it should be.
As for the combination, I would be happy to go with the one the works seamlessly.
In the long term, I would look to relunctantly decommision all my SSG kit once the WatchGuard appliances have bedded in.
I will report back once I get an actual unit with an update.