Let's see...
Fair Walda - Seems like a good bet. Tell it Walder Prime as it is. If she looks good, has a sharp wit and magic on top of that, then various Essosi will throw money at him to get that marriage.
Robert - Also seems like a good bet. Citadel pretty much means he is dead as far as his house is concerned, so this is automatically a win for Walder Prime.
Malwyn - Same and he already has some basics. Might have Devil contacts though, so keep an eye on him.
Tyta - Eh. Test her. You don't need to be smart to be a Sorcerer or Druid.
Perwyn - We don't really have someone to squire him under.
Willamen - Having a friendly Seven cleric might come in handy and there is always the chance to convert him to a more useful faith.
Olyvar - Seems uninteresting.
Roslin - Same.
Arwyn - As a favor to Walder Black, I would definitely take her.
All in all, let's just test them right here and now and then take everyone with the aptitude. Free mages! We can offer the remainder a good education in the Deep and for the more martially inclined, a career in the Legion.