
DualJay |

Aureate |

How does the feat interact with the spell? Do you get one more at the max duration? Raise the limit from six to seven horses? A free extra summoned horse?
Mount, Communal says that you summon up to 6 mounts and divide the duration among them in two hour increments.
Superior Summons adds one to the number of creatures summoned any time you summon more than one with a summon spell.
So, if you summon more than one with Communal Mount, in this case I'm guessing you would be going for the max of six, you would get an additional mount. In this case 7.
The duration is divided as you choose amongst the 7 mounts in 2 hour incremements. Nothing in Superior summons is giving you a bonus to the duration. This means that if you want 7 mounts for the minimum 2 hours you would need to be a 7th level caster.
At least, that's my reading of it.

Dave Justus |

One thing to be aware of is that mount is significantly different than summon monster. A mount doesn't automatically attack your enemies for example. The spell says it serves 'willingly and well' but serves is modified by the previous sentence be 'serves as a mount.' It is reasonable to rule that a mount that isn't being ridden will either just wait or perhaps move away if threatened.
If you have clever combat ideas for this make sure your GM agrees with you on how it works. In my games, mount is effective at what it is designed to do (provide transportation) but not very effective in creating a herd or rampaging stallions to charge down your enemy.
That said, clearly communal mount would allow you do have a maximum of 7 mounts with superior summons. What that does to the duration of the spell is less clear. I could make an argument that a sixth level caster summoning six mounts for two hours each gets a bonus mount with the same duration because of the feat. I could also argue that the extra mount counts as part of the total for dividing out the duration. Order of operations isn't clear for this situation in the rules. I would probably have it be in the way that was most favorable to the PC.

Aureate |

One thing to be aware of is that mount is significantly different than summon monster. A mount doesn't automatically attack your enemies for example. The spell says it serves 'willingly and well' but serves is modified by the previous sentence be 'serves as a mount.' It is reasonable to rule that a mount that isn't being ridden will either just wait or perhaps move away if threatened.
^This^
If you want a bunch of horses to do combat, use Summon Monster. If you use Summon Monster IV with Superior Summons you get 1d4+2 Horses with the Celestial or Infernal template applied. Yes, it's a higher level spell, but they will follow attack commands.
But all that said, if you are GMing and doing this for some cinematic reason, then go for what makes the most sense for your story.