All but the main eido are just there for stats and their summon. Same ele eidolons get 10% more stats. Mostly pick for summons + decent stats and just add the 10% on top of any same ele ones when comparing.
Weapons, ultimately yes, you'll want a grid of 10 same element assault weapons. At start though, you'll mostly be looking for stats, so an off element SSR is waaaay better than a same element R. Element only becomes relevant when leveling weapon skills, which is generally only worth it imo for: max break assault/pride SSR, max break assault SR, lower break dual skill SSR (assault + defender). There are ofc always some exceptions that might be worth it in certain scenarios, but take that as a rule of thumb. It takes a lot of time and skill level fodder to really upgrade all grids, so it's not too wise to waste them.