A weapon gets pushed out of the grid once you've got enough SR/SSR weapons with assault for whatever your main composition is.
An eidolon gets pushed out when you've picked 5 other summon effects ahead of it and/or you've got enough SSR eidolons of matching element for the +10% stat bonus. The summon effect seems more important to me, though. Especially since eidolons count as their own 'frame' for buff/debuff purposes.