Okay, when will this take place? I haven't been to Skellige yet but already Level 19 from grinding quests.
Be free to go to Skellige.. Even to go in Kaer Mohren.. Until You get quest "Brothers in arms - Novigrad", You can procrastinate it.. But, to complete this quest, You must complete Triss quest line..
And don't worry - game itself will Tell You in specific point, that "If You continue, You cannot complete some quests anymore".. (it will tell You during the Isle of Mist)