I think all silver summons are food, yes. The exception seems to be the Angelmon which is food but needs to be leveled up to provide max xp (think snow globe from p&d) and can be awakened to provide the bonus XP to same type.
I haven't kept any of my other gold summons, like the Imps or Vikings. Maybe that isn't a bad idea, who knows at this point.
Right now I'm running 2x Water Fairy max level, Fire Hellhound, Light Garuda, Wind Golem. And I just pulled a Elucia (awoken Water Fairy). So as soon as I awaken my one I could just have 2x Elucia. Everything else is waiting for 4* evo but can't get the multiple 3* guys to do 'em.