They may run afoul of the Tholians. Tholians are very very very territorial. And if they are scouting as you say. Then that happens sooner than later.
Webway makes them untouchable
How by chance considering Tholian webs kind of affect dimensional barriers.
What's the SOP for ST big players to do when they meet some new, advanced civilization that doesn't fuck around and wants to be left alone?
The estimates i've seen are between few dozens and few hundreds.
Yeah, it's very imprecise. We have all the named ones like Biel'Tan, and an unknown number of others. All we know for certain is that the Eldar have been a dying race for millenia...supposedly.
Most Eldar are not interested in wars just for the hell of it, though they can have rather liberal definitions of someone fucking with them.
Yvraine yes, cammoragh is not here
How would battles go down?Well it's hard to judge, but if it was just a "number" of Craftworlds showing up for some reason, it could be interesting as we'd have a finite amount of impressive quality resources to call upon. Since they aren't the top dogs... and remember their history, they would be less likely to fall into their depraved debauchery one would hope. Plus each Craftworld is distinct in their culture and society and the like which is worth discussing.
Plus you'd have troupes of Harlequins potentially dancing about. Plus you'd get the interesting tales of Asuryani Outcasts who just want to strike out independently with much less reduced worry or concern of falling to corruption which really opens up their options when it comes to becoming amoral mercenaries and corsairs.
Eldar tech is half psychic bullshit, and their escort and scout ships make a Sovereign class look small. Who is even going to try fight them? Anyone sane would rather go fight the Borg or something.