I do agree, having them all in a single special that's 1 hour to 1 1/2 hours long would be too crowded. However, if they were to do a few in the Anniv special, and then carry that story over to the Christmas Special there ould be room storywise. Or.... it could have been planned to have a Series long arc, with a single previous Doctor per episode, change the Doctor each episode, or rather than continuing in the Christmas Special start it with the episode or two before the Anniv Special and conclude the storyline in the Anniv Special and that would give you two or three episodes to spread hem all out in, etc.
Thee's any number of ways they could
have done it, and doing 1 extra Doctor an episode is less risk because you wouldn't be throwing 5 or 6 or however many previous Doctors at the audience at once, that most are completely unfamiliar to the bulk of the audience.