The classic JL roster is usually Superman, Batman, Wonder Woman, Flash, Green Lantern (Hal Jordan), Aquaman, and Martian Manhunter. Hawkgirl has been a member, although Hawkman was introduced first. Nevertheless I doubt Gunn feels beholden to entertaining a specific roster.
Put most anyone else in the role of Jimmy and I’d have found all of the women fawning over him kind of annoying and unnecessary (maybe this is canonical for the character in the comic books - I wouldn’t know) but because it’s Skyler Gisondo I’m like lol yes ladies you are so right for that
If this continuity handles “the Justice League” the way Marvel Studios handles “the Avengers” (as the big crossover event, not an actual team that exists in universe as an ongoing organization), then I don’t think anyone in the “Justice Gang” besides maybe Hawkgirl makes the cut. In that case, I’d assume the initial lineup being Superman, Batman, Wonder Woman, Stewart GL, some Flash, and two of the following: Hawkgirl, Aquaman, Martian Manhunter, Cyborg, Black Canary, Zatanna. However, I also wouldn’t be shocked to see the Maxwell Lord funded “Justice Gang” get an in universe rebrand to “Justice League International.” The pieces are there for a take inspired by that era (down to some overlap in cast, though I’m sure it wouldn’t be one-to-one), and it fits Gunn’s style. Honestly could even see that being the next Gunn-directed film.
(Gosh, I haven’t cared about brainstorming cinematic universe shit with comic book knowledge like this in so long, lol.)
No, he is usually portrayed as a bumbling but well-meaning nerd. It could be different in more modern media like Smallville or on the Arrowverse shows, I wouldn't know.
Or he could play a small but important role in kick-starting an interdimensional planetary war...would love to see the New Gods down the line