So reading over the Master Summoner and it says that you can cast summon monster 1 as a spell like ability equal to 5+Cha mod times per day. It also says that there are no limit to how many Summon Monster 1s you have out on the field, however if you summon your Eidolon then only one summon monster may be in effect while your Eidolon is summoned.
Question is does this mean that the first or the last creature summoned remains in play or is it players choice which creature stays and which goes back to being unsummoned?
Also is it only Summon monster 1 that he has uses equal to 5+Cha per day as the spell like ability or does he gain that ability for each Summon monster up to 9 as he continues to level?
All of the summon monsters use the same pool. In short if you can summon 9 times per day then even if you summon a level 1 monster it counts as one of those summons.
As to which monster gets to stay if you summon the eidolon and a monster I would let the player choose, but I don't think the book says which way it goes.