If you have the Magus Sisters already, you should not have ANY trouble with both of the forms. However, getting them is a time consuming process, you can't control them directly, and you will have to train anyway, because their stats are suckage when you get them (unless Yuna has completed hers and at least one more section of the grid).

In short... totally worth it.

But really, you should not have too much trouble with using Auron/Tidus/Yuna; as long as Auron knows Zombie attack, you won't need Celestial Weapons (you really don't). Since the Pagodas heal Jecht, raise his Overdrive, and null any statuses you inflict on him, the best option is to leave them alone. If you kill one, then its HP will raise by whatever amount of damage you inflicted on it to kill it, and its HP will continue to raise. (First one killed only, second one killed does not receive bonuses).

The key to surviving is Protect and Haste. Nearly all of Jecht's attacks are physical, so cast Haste on Yuna first (or Hastega on all if you have it, which I assume you do). when Zombie'd, Jecht also takes major damage from curative spells, so throw X-potions if you have them while he is Zombie'd. (but not the Life spell, so don't bother with that or Pheonix Downs, when Zombie'd, he begins to glow an alternating green/normal colors). Also, his Overdrive will not charge the first time he is "healed" by a Pagoda when Zombie'd.

If you wish to use Rikku's Trio of 9999, then Pray and all attacks will do 9999 (but not more, if they cannot already surpass 9999 damage). Lulu , Kimahri, and Wakka are non-factors in this battle, unless you can consistently disable the Pagodas, without going out of your way. This means Ultima, another time consuming hassle to get (in this case, not worth it). Remember to heal Petrify when it occurs.

With Tidus' "Talk" command, SeeD already covered it. Use it when Jecht's Overdrive bar begins pass the initial orange part (about 5/6ths of the way). It also delays Jecht' turn by 1.5 turns. His Overdrive WILL kill you if you ahve less than 9999 HP, so it is easier not ever having to see it (but if you want a description, it looks like Vivi's Meteor [FFIX], crossed with Lulu's Ultima in outerspace), before it hits you for UBER WTF PWNAGE damage. If it is, then hope Yuna has time for a turn, and summon an Aeon to take the hit.

With this knowledge, you should not have ANY problems fighitng Jecht, even if you only have at most, 3500 HP (I wish someone had told me about all of this crap...). Weapons are also a non factor unless you can do more than 9999 without Trio of 9999. (But using whichever ones you choose should be fine; Armor is VERY important, if you can, equip with Stoneproof/Ward, +HP/+MP, and the fourth slot is your choice, Auto-Protect, perhaps)