Melia has a +15 Agility skill that takes very little points to learn, link it to everybody. Enlightenment (last skill in the same tree) is important to be able to hit the post-game enemies with ether attacks (just in case you want to try fighting them). Sharla likes Melia's skills that boost ether.
Riki has really good skills to link. He has skills that gives the same Exp and AP to characters who aren't fighting. He's the only character with skills that increase the chance of getting gold and silver chests, which makes getting crystals and gear way less annoying. He's the only one with cooldown-reducing skills too (those are really good as well).
I played with Riki and Melia a good amount of time to have them get more skill points, and they were able to learn all their skills minus one.
All the quests in Alcamoth are timed. You have a good amount of time to do them, though. After you've finished with Prison Island the quest to unlock the 4th skill tree for one character will be available somewhere in the Eryth Sea. You'll have to do a few others first in the same area to unlock it, though.
