There you arrived to actual propaganda. Just you should remember one thing - peoples will look on your poster short time, so you just should place info in short, loud losungs, which will rememberable or memetic. Secondly, your poster could/prefereable be rememberable by design (color scheme/design of elements on it, like "Keep calm" for example). Main purpose - create interest as much as possible, and be rememberable (if person who look on it dont show interest, he/she should at least remember "oh man, i saw Core looks for recruits").
Memes not always ooRP or even funny thing. That basically just something memorable, which can be used for increase content depth by references on something well-known by some auditory.
Also it can be even design decisions, for example we widely use Hurricane silhouette in our things, cos Hurricane most impersonating of Coalition fleet might + that design with reference to old Sol Coalition. (yes, there some recursive layers of memes, that like nazi uniforms, even ships and soldiers look can become a propaganda tool)
For Core main meme obviously DOLOR TRANSIT GLORIA AETERNA EST, Makos swarms photos, maybe some conflicts like Toledo battle or Zoners dramas (I'm obviously not expert in Core history). Basically, any really big rememberable conflict, or any rememberable inRP person (you have that Implosion's char as rememberable face for Core) can be inRP meme. Propaganda can also just create its own forced memes after all.
(I personally for example used Plesetsk siege (even if it was waste of Coalition and Sairs resources inRP mostly) as kind of Stalingrad meme, because it was a thing inRP, and that sort of "Overcome battles without chances" very viral for post-USSR auditory. Well that kind of things exists everywhere, because when your soldiers lose in heavy battle, you have only one choose - make them heroes before enemy will make them fools.)
Yes, another things, by choosing of propaganda direction, you can sort out preferably auditory, peoples of compatible mind-set, that need keep in minds too. (Well, concretly that not much a thing on disco, cos small peoples amount in general)
Which one? photos of Makos swarms can be "redacted" photos with target create seems of large fleet. Gloria aeterna official inRP losung and used much, all noticeable characters exists inRP too. Battles and zoner dramas existed inRP too. Some cultural references will be saved after 1000 year too( like right now we use antique memes, because they was wrote, and saved.).
I guess no need much hesistate mix slighty ooRP and inRP, while end-product will be inRP. ( i mean, for example КФШВ ПФЬЬФ УМУКН ВФН really ooRP, while GLORIA AETERNA inRP. Swarms can be just propaganda enlargement, that propaganda and it can lie after all. That sort of art. Art show reality in angle, benefitiary for propaganda maker. (Well, honestly i very often notice, peoples very avoid use lie inRP, while it normal for information wars and net channels).
Also ooRP memes can be remade inRP. For example instead of "SPAM MAKOS" use somethin like " SHOW THEM MIGHT!", "INCREASE MAKOS PRODUCTION!", "NEW MAKO - NEW PUNCH TO CRIMINALS", "ALABAMA SHIPYARD - THAT BABYS WAIT YOU". In short - most of ooRP things can be reverted in inRP with saving of memetic impact.