The thing that bothers me is Felassan. World of Thedas makes it clear he is an ancient elf, not just a modern recruit. At the end Mihris says: "Will you take me to your clan?" To which he replies: "I don't think you want to meet my clan, da'len". You could just argue he is being evasive and he just doesn't have a clan. However, that emphasis on da'len is given in the book. It's literal meaning is "Little child". By this point Felassan is not thinking of modern elves as non-People, hence his support for Briala and his willingness to die for it. However, he does still think of them as children compared to himself because he is ancient and so are his clan.
You have to ask yourself, what did happen to all those followers of Fen'Harel? Did they just age and die? Or would he have taken steps to ensure they were protected and preserved until he could return and restore them to what they once were? His words to Abelas seem to confirm the latter. "Your People yet linger." When Abelas says: "Elvhen such as you?" Solas replies: "Yes." Abelas confirmed that he does not regard the Dalish or any modern elves as his People. So Solas is clearly not simply referring to a group of modern agents when he says this. I also think he is absolutely telling the truth to Abelas because Abelas would sense if he wasn't. Their numbers may not be huge compared to what they once were but still could be sufficient to be spread across Thedas, particularly if many of them, like Felassan, had been instructed through the Fade to awake from Uthenera and go out into the world to prepare for his return. Why should we assume that Felassan was the only ancient elf out their working on behalf of Fen'Harel? It would also account for his words: "The People need me," because only he can undo his magic and his determination to go through with it is because time is running out for them. Abelas confirmed that each time they awake there are fewer of them. As the years pass, more and more pass beyond the point that the Fade will sustain them in Uthenera. Solas needs to take action to save his People; the ones he led to freedom back before he created the Veil and any other ancient ones still slumbering awaiting the return of their gods.
I don't deny that modern elves may well join Solas and he would have no trouble recruiting among them. Nor do I doubt he would be willing to use them and discard them in order to achieve his ends. Felassan clearly did that with Clan Virnehn. Solas also admit he is quite willing to sacrifice anyone, even friends, and we know he did kill Felassan. Cole maintains this was for the crime of regarding the modern inhabitants of Thedas as People. Solas also says to Lavellan that they are going to die along with all the rest. So if he is recruiting modern elves, it is not with the intention of saving them, at least not in the sense of their present form. In his own mind, if they die and become spirits that can be reborn in his new world, then effectively he is "saving" them from the modern world. I don't think that his recruits would view it in the same way though.
However, the chief reason he might only restrict his followers to the ancient ones who supported himself or Mythal is that they are the only ones he can trust. That trust must have been shaken when Felassan rebelled against orders. How much less could he trust modern elves, particularly like the Dalish who revere the Creators and would not be enthusiastic about any plans of his against them, even if they would welcome their release.
Have the any of the writers commented on this? Also any thoughts on Cilian?