From previous discussions on tactic, i'm always surprised how different some people play.
Most consider Varia a must have, or consider Yarra useless, while my experience couldn't be farther from this.
For example: i can't imagnie using any succubi without buttplugs and vibrators (heh), in which situation yarras and qums ability to raise SP (not to mention mana) of others becomes much more of an asset (and as someone said, its quite fitting how at the start of the combat the ladies "tend" to their master first, so he can unleash his incubus powers). Add to it, that as you said: every resource is limited, even refill items, so yarra being a well of mana and sp means you can keep using your heaviest hits without "wasting" consumables, indirectly raising your damage output without actually attacking (i currently only ever used potions in the orc only battles, and sparingly even then).
As for tactics, i think its much better to start with situations/roles and entire parties, rather than breaking down every character. For example:
-The "sustain" or general purpose party that can deal with any non-boss, non-anti succubus party is simon-riala(until her, its either robin if its mostly single target, or altina if its groups)-yarra-qum: the caster(riala/robin/altina) have all elements to take advantage of enemy weaknesses, yarra can keep everyones mana topped and kick start either simons sp skills, or qums party heals, qum has party wide buff, heal, sp and mana restore, and everyone (sans robin) have AOI attacks, most with disabling effects. This needs buttplugs and vibrators for the succubi, and good staves for the casters (2-3 freezing staff after investing in the ardoheim magic shop for discount, then assault rods from stenai). The main weakness is the glass cannon nature of it: if too many foes remain standing after 2-3 turns and none of them are disabled form the aois, then bad times are ahead, or if the enemy can 1 shot someone otherwise, you have to make an effort to prolong the fight to more than 2 turns, so simon and yarra can use their recharge skills.
-The second is the "boss fighter" group which has the basis of hilstara and carina buffing and keeping everyone healthy (later even simon helps in with better team heals than carina) and either Robin, Varia, or Aka/Vhala depending on what the boss is allergic of: magic, physical, or can be debuffed/ and there are many of them (like erosia temple sluts). The plus side that in an optimal run, assuming you buy every custom equipment, you'll have everything you need for this (except a staff for carina, but thats covered for the above group too). Theres also enough status immunity items, to have everyone of both of this two groups to have one at all times, so you can swap between the two groups at will without item fiddling
That leaves Uyae and naili as my least used memebers, but even they get used when its trash cleaning time for their aoi damages, so for example simon-altina/naili-uyae when i want to keep the other 2 setup in reserve, and just want to burn though everything fast not caring about depleting the mana reserves.