I think that it shouldn't be a problem, provided you only allow the monsters to take a standard action on their first turn. Maybe I just haven't met a really good summoner player or I am missing something, but it seems like the low duration on Summon Monster XYZ means that you can't really cast it ahead of time, while the one-round casting time means that you're rather likely to lose the spell if the action has already started. Outside of combat, the difference between a one round casting time and standard action casting time isn't likely to matter.