The only thing that seems a bit strange about this is that it seems a little random.
I can see a certain logic to it. As already suggested, they can use the actors playing Hartnell and Troughton in Adventures in Time and Space to play their respective Doctors. And with McGann and Tenant you won't have the age issues. Tennant hasn't aged that much since 2009 and we never saw the Eighth Doctor when he regenerated so you don't have to pretend he's not as old as he clearly is.
I'm assuming that while time differential worked fine as a hand wave to explain Davison being older in Time Crash's ten minutes, it mignt be seen as too much of a stretch explaining four Doctors for an hour.
And while they won't have the age issue with Eccleston, they do have his history with the show to deal with.