Yes, typically you'd need to create a scene for an idea, with the scenario editor you can do most of the work. You might want to do some scripting to make the scene more lively. Then you need to create camera paths (again scenario editor) and finally render the thing. I don't see an issue with uploading videos about random topics to the play0ad youtube account if they are of similar quality as the release videos, whether short or long. Civ intros sounds like a good idea. Doing a few of them is also good practice for when the next release video is due in roughly 6 months. Not necessarily recruiting as an official team member but surly as a contributor. I for one contributed to many open source projects, hardly any of them in an official role and don't see a reason not to do so in the future. Less commitment but still able to improve what I already use and value. For some that is plenty satisfying already. 

I would be glad to help out with those, hardest part of making trailers is getting the necessary footage. I'm not sure if your goal is to recruit people to play or work on the project mostly (I mean why not both lol) but you can start Dev Logs, run 1 minute long shorts showcasing civs, promote community maps, etc.

Yes, BUT in our mod "citizen soldiers" are not as good at eco as civilians....  Sparta doesnt have civlians... their Helots have the same gather rates as civilians but are worse fighters than normal "CS" We have the aura based ones... I need to think of how it could work for units that are NOT know for formation, ie the tribal warriors.  Auras need another unit to trigger the aura... So phlanx for example only takes effect if there is another unit with it close by,... even though it really should be you need at least X amount for it to trigger...  I dont know how to do that yeah, so basically spartiates take longer and longer to train as you progress, so they are harder to reinforce so you need at least more Peiroikois.  We actually limit neomades for now, cause they are 0 pop... they are helots who eventually promote and they are weaker than all CS but they don't take up pop... but I may change that a little since that was back when Helots were 0 pop.

I think Sparta and Thebes populations were comparable, but full Spartan citizens were abysmally less. Maybe forget that extra population penalty, and deal with it by limiting more the number of Spartiates as one progresses. Neodamodes units should kind of become a necessity. All this has been proposed already by me and others before me, and aligns well with reality.

Maybe have a look at DE? @wowgetoffyourcellphonehas aura-based formation bonuses such as shield wall for hoplites and massed pikes for Macedonian phalanx.