Hoping to make your life's design a bit more dynamic!

Not all idols are worth combining.

As far as I have seen (having leveled many idols to the maximum level of 60), idol stats do not vary; lowly Soil will have, give or take a couple points, the same stats as mighty Harn. So, combining will only give you a different set of spells.

This seems lucrative at first because the new idol will have a larger volume of spells (except Void); however, this is a problem with some idols. Having a larger pool of spells simply means that the idol will have a lower chance of casting the most effective spell.

Take Soul and Feat for example. These two idols know only one spell: Tornado. This spell is the strongest single-target spell you can access, and these idols will cast it 100% of the time. Now consider the final form idols like Harn, Kali, Siva, Yarn, etc. These idols learn a variety of spells, but they'll most often select a spell that isn't as worthwhile as Tornado.

Additionally, fusing two idols yields an idol of reduced level. For those curious, the resulting level seems to be equal to (FirstIdolLevel + SecondIdolLevel) / 8. This may not be the exact formula used, but it lines up with what I've observed. Not a problem in the long run, but something to consider early on.

The one advantage to knowing many spells... If you're facing an enemy with elemental immunities, your idols are smart enough not to cast completely ineffective spells. This is noticeable toward the end of the game when you start facing enemies who are untouchable by the classic lightning-, fire-, or wind-based spells. Without an appropriate spell to cast, your idols (after attempting to inflict status ailments) will rely on physical attacks--all except Harn, who will chain-cast the non-elemental Typhoon spell. (This is mostly trivia, though, because against such enemies, you wouldn't use idols in the first place.)

Combining isn't all bad. Idols with weaker or undesired spells like Soil or Lore can only improve. Consult an idol combination chart to get the most out of your idols. ritchie's guide on GameFAQs covers most of the valid combinations (one he missed was Siva = Doll + Rage... there may be more).

And... I haven't seen a list of idol spells anywhere, so I've compiled it here:

Soil: Basil
Star: Whirlwind
Lake: Calm
Lore: Meta Pozo
Fire: Fabol
Skye: Basir
Wind: Meta Basil
Dark: Fabor
Soul: Tornado
Feat: Tornado
Love: Fabor
Void: Pozo
Fall: Basir, Calm
Vann: Basil, Pozo
Guye: Fabol, Pozo
Roda: Basil, Calm
Wolf: Fabol, Basil
Doll: Whirlwind, Tornado
Deli: Basir, Tornado
Rage: Whirlwind, Meta Pozo
Kali: Whirlwind, Fabor, Tornado, Meta Calm
Siva: Basil, Fabor, Tornado, Meta Fabor
Toto: Fabol, Basil, Fabor, Meta Stun
Harn: Basil, Fabol, Basir, Fabor, Typhoon, Calm, Stun, Meta Stun
Janu: Basil, Fabor, Meta Basil, Pozo
Yarn: Basir, Fabor, Calm, Stun
Gale: Meta Basil, Meta Fabor, Meta Pozo, Tornado, Calm

I ended up finishing the game with Harn, Gale, Soul, and Feat. Ironic how even Harn's strongest spell, Typhoon, still deals less single-target damage than Soul and Feat's Tornado. Don't lose these two idols!

Edit: Despite fusing Harn, I ended up replacing it with Love in my lineup. I found greater value in a consistent Fabor spell over Harn's spell roulette. In a second playthrough, I left all idols in their original state in order to experiment once I had them all.

That's an amazing amount of data you compiled there, LesterKlarth, thank you!
All of that is highly elucidative and should be very educational to players wondering
what to do with their idols.
Welcome to the forums.

I believe that a version of your compendium is worthy of publishing somewhere as an Idol FAQ, preferably at a venue that doesn't consider console emulation and so-called fan translations taboo to the point that they delete useful information that is offered to enhance the gaming experience. Thanks! We would be happy to publish said Idol FAQ here.

Basically, with idols, I just followed my nose during two complete plays-through in the production phase. It gladdens me to see someone delve into a previously untranslated RPG this deeply.

Thank you for the kind words! It's due to your efforts that I'm able to enjoy the game to this extent. You've made at least one gamer very happy.

I would be glad to put together everything I've learned about mercenary, idol, and spell mechanics into something more formal. If this is OK with you guys, please let me know what would be easiest in terms of submission.

That said, if anyone has more info to share, please continue to post. Another interesting aspect of idols... they are immune to stun/petrification, a bonus that comes in handy as you get deeper into the game.

Ouch!

Void only has Pozo?

No wonder everybody hates him LOL!

Lake is kinda weak as well.

Awesome Idol FAQ lesterklarth

Just PM me when you have a doc ready. I will give you my D-D e-mail address to attach it. We'll add it to the download package with the patch and readme, HTML-ize it, and post it under "Documents" on the home page. We'll also mention it on the project page and include a hyperlink to DL it directly from there. If you want to submit it to that place that deletes emulation posts on their boards, that's fine. It'll always remain your property.