I don't know so much about optimal usage of them, but in my pseudo-NG+ run, they've been my floor trash and boss battle lead party since the start. Couldn't be happier with their overall performance, as they were almost never hard pressed to deal with randoms. That said, my setups for them have fluctuated from floor to floor to suit my needs, so I'll list some of the subclasses I felt worked well. Note that they are NOT the only people I use, so I'm not aiming for a 4-member team review here (I feel that would make Hard Mode insurmountable of a challenge).
Wriggle
Easily the best of the quartet. Stuck to Guardian early on for the improved defenses and damage reduction. Having an additional group-wide defense buff at the start of battles cushioned those times when I just couldn't outspeed something on the floor I was on. That said, hands down, I have not regretted making her a Strategist. Everything about Surprise Attack is glorious: an early Wriggle turn is an enemy party that dies to the fight command because of poison, An early Mystia turn is either a great MP restoration chance, early PSN or PAR chance, or just a quick nuke with Midnight Chorus Master. Early Cirno turns mean the enemy never gets to go. Early Rumia turns sadly don't mean as much, but because she's got such good recovery on her attack cards, she generally gets to go twice before anyone else (my own Puppet Top). Universally, any early turn means immediate Team 9 buffs, which can be a key player in killing some game enders before their apocalyptic turns. On the downside, I feel Strategist Wriggle tends to have some dead turns on bosses when she's incapable of or just not needed to perform dedicated switching.
Mystia
Mystia served me as a mix of nuke and MP battery for the most part. Early on, she used Monk to get preemptive turns and used that in conjunction with Iyashikei to help keep MP levels high on dungeon outings. Still perfectly usable for damage, as Midnight Chorus Master is pretty much unmatched by the other Team 9 members, and Poisonous Moth's Dark Dance still wins the award for best disabling spell in my book. That said, I've been using her as a Magician. Might seem odd, but with free Strategist turns every so often, chances for free spells, and access to Magic Transfer and Iyashikei, she's quite effective at clearing encounters or using non-threatening enemies to get everyone back in top shape. In boss fights, she's always gotten a lot of flack from me. I find it impossible to switch her into anything because of her frailty, and she does almost no damage for me. Monk with the defense piercing card might be a good swap for that, but I admit I've almost always had something in my back ranks to deal with whatever bosses stop Mystia cold.
Cirno
Probably the hardest to place here because of how often I've had her be dead weight. Cirno really lives and dies by her ability to disable things. For randoms, she's just fine. Serviceable damage, exceptionally fast, capable of disabling heavy threats for the most part. That said, for randoms, I've been using Transcendent just to make her a better slot 2. For bosses, I generally make her whichever offensive class offers a spellcard that hits weaknesses; her naturally composite nature works to her advantage here. For recent example, making her a Monk made the 20F West waypoint boss an absolute cakewalk after being walled several times in a row by it.
Rumia
Sorceror. I've tried a few things, like Enhancer, Healer, and Magician, but she just doesn't have the turns for these sorts of jobs. She's an amazing multi-target healer. She's one of my strongest sustained damage dealer with Moonlight Ray, and appreciates the small bit of elemental diversity in Penetrator. She still manages a respectable MND almost on par with Wriggle despite less investment. She definitely also appreciates the MAG buffs from Magic Beating and Mind Assault. Gambler might make for an interesting choice, given her fairly low MP costs and quite good initial damage output, but I've always found her pretty sturdy and I wouldn't want to throw that out for some impressive-in-practice boosts to her damage.
