This is for 5E mostly, but it applies well to all of 40K.
One of the easiest ways of finding out whether or not you’re about to play with someone who’s chill and wants every player to have a fun game is to suggest a tiny modification of your faction’s rules. It can be as simple as altering the unit size cap slightly, taking a cheap piece of wargear for free or adding an optional upgrade, or adding or changing a special rule, or any combination of them to better represent your faction’s fluff.
On the surface it can sound really power-gamey, but that’s not what I’m talking about at all. In 5E lots of factions remained stuck with codexes from 4E, and some codexes (in particular the 4E CSM one) are pretty damn bland considering the variety they’re supposed to represent. There’s also the issue of convenience; look at the Shining Spears Aspect Warriors - their official GW kit comes with 3 Riders, since it’s basically a squad of Guardian Jetbikes with some failcast bits to convert them to Shining Spears*. If you buy the Ghostminis kit instead (which I plan on doing in the future), you get 6 models. Their army list entry caps the unit at at 5 models, but I’m going to run a unit of 6; any adult that makes a big issue out of this probably won’t be someone I enjoy playing against, especially since the 4E Eldar weren’t power houses and quite frankly got eclipsed by many of the 5E codexes such as BA, SW, GK, IG and Newcrons.
* Interestingly, at least on the Swedish page, the Shining Spears cost as much as the Guardian Jetbikes, so you might as well grab the Shining Spears for the extra bits if you’re getting Jetbikes at all.