Look here for summoner guides/discussions: http://ffxi.allakhazam.com/forum.html?fjob=15&mid=1188823663309266438&page=1
You should try to get them all, but no one will blame you if you can't get Fenrir, Diabolos, Odin, or Alex right away.
You can solo all of the other ones as a level 20 SMN. Here is a guide to doing it: http://ffxi.allakhazam.com/db/jobs.html?fjob=15;mid=1092674588861471843;page=1
Or if you have a higher level job, you can solo the harder versions of them. Anything 75+ should be able to solo or at least duo them. I would assume Fenrir is easily soloed with the new level cap now, too, especially since some jobs could solo or duo him at 75, but if you don't have a high enough job, you will need help. Start working on CoP/ToAU missions so you can eventually get the other three. Odin, especially, is a lot of fun in abyssea.
For just about any mage job 82+, you won't go wrong with /RDM. At really low levels, go with /WHM. Eventually, /SCH becomes better than /WHM with all its job abilities that help save and regenerate MP and the ability to cast aspir.
At the current level cap each of these subs offers:
RDM: stoneskin/blink/phalanx, refresh, convert, single-target bar-spells, Cure 3
SCH: light/dark arts, strategems, reraise, sublimation, status cures, drain/aspir, sleep, cure 3, weather spells that can be paired with gear like our relic head to reduce perpetuation costs
WHM: stoneskin/blink, divine seal, aeo bar-spells, cure 4, auto-regen, reraise Edited, Feb 26th 2011 5:40pm by RizzoRazzle Edited, Feb 26th 2011 5:41pm by RizzoRazzle