You'll start out with basic espers (when you get them; about 6 hours into the game), get everyone to master one esper each immediately. Then later as you gain espers switch them to suit your needs.

Espers aren't permanently equipped, and you'll have plenty of time before the final battle to really hit the espers hard.

Relics are based on preference, really, but it's logical to have people good in magic to use magic and equip relics accordingly, as well as having tanks tankified by relics.