I don't see how the new young Xmen roster could be the ones who meet the FF, unless of course they meant the aged versions of the characters but I doubt we will see the original actors from X-Men return for that. FF seems pretty firmly rooted in the current world, so they would need to interact with the older cast, unless some kind of time travel/world merging occurs in Apocalypse (I wouldn't bet against it). I'm not in the "I hope Fox fails" camp, because I REALLY don't think there's room in the MCU for the Xmen/FF without bumping things that have already been planned for. I was worried enough about the addition of Spiderman and the effect he may have on the Phase 3 lineup, so I can't imagine what another "homecoming" would do, especially one that big.
Either way, I'm interested enough in the new F4, and the Xmen movies are solid for me, and I'm excited to see younger takes on Jean, Storm, and Scott.
I can only see them using the portal device to bring x-men to the F4 world, or send F4 to the x-men world. Seems the only way for them to use either the old cast or young cast without any problems. Time travel is possible in the X-world and dimensional travel is possible in the F4 world, shouldn't be too hard to say they meet at some point. We've only seen secluded parts of NY,a part of SF and a part of Tokyo in the X-world, so there's a lot we haven't seen and the 4 can maybe already exist in their world. But I hope its dimension travel and a brief crossover and not a constant thing that they do, just a shared world would be good enough