How realistic is it to be able to complete all the nms for aeonics with 6-8 people with without nirvanas or idris?
Very realistic, you'll need some HQ gear but nothing too expensive. For most of the NMs, you have countless options and should just go for a balanced setup with whatever jobs your players like if you're not overgearing the SMNs.
For the i150 stuff, you'll want something like:
SMN/RDM x3-4
RUN
COR
GEO
For albumen and vinipata, you'll need to add in the bard. They really don't need much besides capped skills, since their only job is to N/T sleep adds.
If someone has a mule(even same account) with access to reis/ruaun, you can use that for a COR and save a slot, otherwise you probably want that 4th smn to offset the COR needing to be in party.
None of the RMEA you mentioned really matter except idris and nirvana, and you can do fine without them. Just make sure your SMNs are otherwise solid. Bard doesn't need gjall or 4 songs either, you really only need NT sleep for the adds on albumen/vini, and you can do that basically naked.
As far as smn gear, without nirvana you'll want to at least polish off a basic set, something like:
Gridarvor/Elan Strap+1/x/Sancus Sachet+1
Apogee crown +1-B/Smn collar+1 or shulmanu / lugalbanda / gelos
af1+2 or +3 / merlinic dastanas / varar+1 / varar+1
campestres(petacc+30 petatt+20)/incarnation sash / apogee slacks+1-D / apogee pumps +1-B
is more than adequate to win these fights with 3 SMNs in a party of 6, or 4 SMNs in a party of 8. Note that all SMNs will need job master.
For everything except zerde, the strategy is:
-Prebuff at entrance with crooked 11 beast/drachen roll.
-SMNs use Akamochi or Shiromochi depending on vorseals and blessings (can get a feel for whether your SMNs need shiro or not pretty quick).
-Proceed to ???.
-Pop NM, sleep adds if applicable. GEO applies bolstered frailty, torpor, and dia2. If COR in party, they light shot dia2. During this time, smns should be assaulting ramuh and RUN should be doing flash/foil for basic amount of hate.
SMNs all astral flow, astral conduit, spam volt strike, convert when out of MP. RUN uses odyllic subterfuge as soon as SMNs astral conduit so that they do not risk avatars being amnesiaed/stunned/etc depending on NM.
NM specific notes:
Vinipata - Zzz. Will die even in phys resistant mode because blunt bonus is huge and his HP pool is garbage.
Teles - Zzz. Nothing special to note, volt strike stuns prevent all the bad moves.
Albumen - Highest DPS check, highest evasion. You can prebuff with scherzo and earthen armor in case you fail to sleep adds before mijin. This'll be the biggest test of your SMNs. Fan out in a circle around ??? before pop so you don't risk all the ramuhs being in front and getting guarded.
Schah - Second highest DPS check, but simple execution. If you can't win schah, you need better gear or better spam.
Erinys - Nothing that interesting. COR or RUN can sub smn to throw a mewing lullaby out halfway just in case it gets TP. Not really needed, should die before it TPs.
Onychophora - May not want to zerg due to absorbs. Totally harmless if one SMN alternates volt / mewing and the others spam volt without 1hrs. GEO is enough healing for an even halfway competent non-epeo RUN or non-burt PLD. Can do pretty easily with non-SMN setups too.
Warder of Courage - Easy. Medium DPS check, be quick on odyllic so avatars don't get stunned.
Kirin - Best to only use one conduit on first form, and save odyllic/other conduits for Kouryu. Steadfast tonics for kouryu. May be some risk of loss outside your control if avatars are terrored. Easy pops to get anyway, though.
Zerde is the only one that requires significant change from that setup, it can still be done with 8, but will be harder. You want a stunner for the worst moves, and 2 GEOs. Something like:
SMNx3
COR
GEO (languor torpor)
GEO (frailty malaise)
rdm/blm or sch/blm (stun tp, one of the geos should entrust focus)
run
You'll need to change your apogee to path A and otherwise try to make a workable flaming crush set. It's pretty easy to hit 99k flaming crushes, and 3 SMNs will be enough for 8 people if everything else is controlled.