I'll start this off by saying a lot of your choices are team dependent ones in that some of them are tanks that can also be used as attackers and vice-versa.
My subclass suggestions:
Rinnosuke: Appraiser (Not a very useful buffer nor healer. Appraiser's skills work very well in conjunction with Rinnosuke's.)
Rinnosuke's healing is quite good late-Plus when you can max one of High ATK/MAG boost, and even better if you max both. Especially since buffed Rinno's healing is even stronger.
His buffing isn't that good, yes, but Enhancer gives him a 35?%ish DEF/MND buff to a single target on 7000 delay along with 15% to ATK/MAG/SPD and being a heal. It also makes Precise Information do the same thing but AoE and 5000 delay, though the heal is percentage. Unfortunately Heart of Prayers (Whichever one is healing a % of HP on buffing) doesn't work with anything besides Battle Command.
Personally I prefer Magician though, even if it gives him less to do than spam First Aid and Form Changes, since he'll never have to concentrate.
Youmu: Swordmaster (What else? She wields two swords and she even has a skill named Swordmaster's Spirit.)
I don't know for sure on this one since Youmu doesn't get that much of a benefit out of having more spellcard variety and more-so passives, but the one passive Swordmaster has might be excellent with her Awakening since it's a large base ATK boost. Personally I prefer Monk, but Guardian is good as well and again her Awakening might change things massively. After all you want to keep her at 1HP so Monk's regen is not that good.
Komachi: Monk (The Shinigami's Work coupled with Area Normal Attack makes her a boon for random encounters.)
Monk is great yeah. I like Warrior to make her Cold spell better but that's a team dependant sorta thing.
Mokou: Guardian (In the plus disk version, Mokou aquires the Sheer Force ability, which ignores an enemy's elemental and status affinities. This means that she is guaranteed to shock an enemy using shield bash.)
Aya: Dive (EXTREMELY OP!!!!!!)
Sheer Force doesn't completely ignore Elemental/Status affinities, it's just ignoring 1/3rd for Ailments. Which is still great -- she can land Shock on a lot of things and her ATK/MAG debuff gets better. She can also subclass Herbalist to make use of that 4MP regen if you're going the tank route. Attacking Mokou prefers Monk overall I think.
Diva is nerfed in Plus Disk so Diva Aya isn't that good anymore. I haven't seen it tested since then though so yeah. I like Warrior on her, personally, but there's a lot she can do thanks to her high speed. I've heard she makes a neat Healer.
Patchouli: Gambler (Patchouli already has crap defenses but the highest magic stat in the game so gambler is a very sensible choice. Great Library's Infinite Repertoire and High Stakes cancel each other out so you won't have to worry about MP reserves.)
It's team-dependent, if you have SDM crew her bulk becomes quite strong with the bonuses and base-stat increases you can get in Plus Disk. If you have Eirin too she can tank a lot of stuff. Gambler is great otherwise though, absolutely. Her being bulky allows her to stay out and utilize High Speed Aria well, which I enjoy.
Satori: Transcendent (I couldn't think of any better subclass so help me out.)
Again, team-dependent. I use Sorcerer in my current run where she can copy Gathering and Dissipating, Deadly Swallow-tail Lance, Hyperactive Flying Object and Moses' Miracle for a strong boost to those along with the standard buffs Sorcerer gives. Transcendent works as well and I'm sure there's a lot of other options, such as physical ones or Archmage, which I might use once I get to Plus.
Yukari: Archmage (Archmage's self-buff will last longer in conjunction with First Category Perpetual Motion Device.)
If you use her as an attacker Sorcerer is way better. It makes her WND attack insanely good and she'll permenantly be at +100% MAG with a strategist out. As a tank/support, Strategist is the only option I've found that fits. She doesn't need more spellcards and she usually sits out due to her amazing passives.
Chen: Hexer (This is a pretty gimmicky setup. Chen's self buff boosts her Attack up, but causes her defenses to plummet. Combining this buff with the Hexer's Conversion allows Chen to recover HP and MP each turn proportional to the strength of debuffs on her.)
It's definitely gimmicky, but it's not that good unfortunately. On the first turn of having -50% DEF/MND she'll restore 2MP but every turn after that she'll only restore 1MP. It's much better to go Warrior to make her damage absolutely bonkers on Phoenix Spread Wings and also to give her less ATK buff decay, which is also really important for her.
Iku: Hexer/Toxicologist (I can't decide between the two. Help me out.)
Suika: Warrior (I can't think of any other subclass.)
Iku as Hexer doesn't work too well due to the same reasons as Chen. Especially since she only has a 70% chance to land each debuff once subbed Hexer, and it reduces what you can use as equipment on her due to losing out on all ailment resist items. I don't know why you'd go Toxicologist, but attacking Iku can sub Monk for all-target normal attacks, Sorcerer for single-target ones, or Enhancer/Herbalist for support stuff.
Suika should almost always sub Monk, it fits her wonderfully as a bulky attacker.
Utsuho: Sorcerer (Second highest magic stat in the game. Increase it as much as you can.)
Transcendant gives more MAG than Sorcerer does, and while you don't get buff upkeep Utsuho doesn't really need that with how much delay her attacks have. You can just re-buff her if you need to. It also makes her significantly tankier, which is a huge boon for Utsuho.
There's a few more in there that are just choices or differences on running a character as an attacker or tank/support but I'll leave them be since this is already getting too long since I got carried away.
