Well, considering they are enabling Teams by default for everyone soon, if they haven't already I think you should be fine. Your federation etc. won't work with Teams in Island mode since your on-prem, and probably calling, but Teams for Collaboration purposes, VoiP and Teams chat should be good.
One thing I would check on thou if you have Skype for Business Online licenses already enabled and the users have been online provisioned from that or not in a hybrid state? If you don't have those enabled then turning Teams on won't do anything since the users haven't been enabled till the Skype license is on and then you might want to look into Hybrid setup scenarios first before hand.
Users have the Skype for Business online SKU switched off and never been enabled to my knowledge.
Teams SKU's are on.
All the users are on the on Premises Skype 2015 and we intend to keep it that way. Complication in location of tenants etc.
Just the main On//Off switch for Teams is off. Although twice now Microsoft have said they would remove it and haven't yet. Strangely enough if someone outside invites us as guests we can get to their tenant ;)
That is strange, I bet they changed that because that used to be a huge issue with inviting guests and if they didn't have teams enabled or licensed you couldn't access other tenants.
As for turning Teams on, you won't be able to use it as accounts won't get created with the Skype Online license not applied, those two go hand in hand and doing so might cause issues, why I was saying you might need to dig deeper into Skype Online / On-Prem hybrid to make sure nothing breaks with that getting enabled etc.
Unless something has changed since with the way licenses work now but that was a pre-req in the past for Teams.
Anyway, if the users don't matter right now, just turning on Teams itself shouldn't effect anything. Your licenses is what will control all that.