Likewise even if it could be done prior to Priority: Citadel II their inclusion would all depend on their character state. So Mordin would be alive if you could do it before Priority:Tuchunka. Thane would be alive before to Priority: Citadel II but still sick and dying and unable to do anything other then sit on a couch. Likewise Legion likely won't show up because he is a prisoner, of the "reapers". Tali could probably still show up (prior to her missions) but likely won't because of the Quarian war and her being needed there rather then a Vacation at a citadel casino.