Flaming Sphere is actually a good comparison, although using the actions on a one for one basis from one through thirty doesn't seem right to me. It would have the advantage of being able to summon 'real' creatures at all levels instead of having to summon Minions or Half HP creatures.
Now I think about it again, this method is probably superior, so here is the revised version of the Summon Monster line, and the Summoning Power Type. (As 4 Minions are seen as equal to one monster from the challenge side of view, I am assuming that having the Wizard trade his actions for those of more than one Minion is also not unbalancing.) I've added in the progression which allows the creature to take actions without the Wizard sacrificing his own to show his control becoming more powerful at higher levels, and because I felt that a higher level version of the Power should do more than simply give a higher level creature.
Summoning Power Type:
A Summoning Power brings one or more additional creatures into the fight. Summons are completely controlled by the character or creature that brought them into existence and act directly after it in the Initiative order. The same character or creature can only have one Summoning Power in effect at the same time. If it uses a second one before the first has run out, the creatures created by the first one immediately wink out of existence. For a Summoned creature to take actions, its Controller must use up the type of action it wants the creature to take to direct it. (Ie. If a Wizard wants his Summon to make a Basic Attack, he must spend his own Standard action directing it.) When a Summoning Power creates more than one creature, the Controller needs to spend only one action to make all the creatures perform that action.
[spoiler]
Summon Monster I
Wizard Attack 1
You summon a minor creature to keep your enemies at bay.
Encounter * Arcane, Summoning
Standard Action Ranged 5
Effect: You summon one creature of up to the Wizard's Level or three, whichever is lower. This creature lasts until the end of the Encounter or until it is slain.
Summon Monster I
Wizard Attack 1
You summon a minor creature to keep your enemies at bay.
Encounter * Arcane, Summoning
Standard Action Ranged 5
Effect: You summon one creature of up to the Wizard's Level or three, whichever is lower. This creature lasts until the end of the Encounter or until it is slain.
Summon Monster II
Wizard Attack 3
You summon a minor creature to keep your enemies at bay.
Encounter * Arcane, Summoning
Standard Action Ranged 5
Effect: You summon one creature of up to the Wizard's Level or seven, whichever is lower. This creature lasts until the end of the Encounter or until it is slain. This creature may take a single Minor action each round without the Wizard giving up his own.
Summon Monster III
Wizard Attack 7
You summon a minor creature to keep your enemies at bay.
Encounter * Arcane, Summoning
Standard Action Ranged 5
Effect: You summon one creature of up to the Wizard's Level or twelve, whichever is lower. This creature lasts until the end of the Encounter or until it is slain. This creature may take a single Minor action each round without the Wizard giving up his own. Alternatively, you summon 1d3 Minions of the same level.
Summon Monster IV
Wizard Attack 13
You summon a moderately powerful creature to fight your enemies for you.
Encounter * Arcane, Summoning
Standard Action Ranged 10
Effect: You summon one creature of up to the Wizard's Level or sixteen, whichever is lower. This creature lasts until the end of the Encounter or until it is slain. This creature may take a single Move action each round without the Wizard giving up his own. Alternatively, you summon 1d3+1 Minions of the same level. These can each take a single Minor action each turn without the Wizard giving up his own.
Summon Monster V
Wizard Attack 17
You summon a moderately powerful creature to fight your enemies for you.
Encounter * Arcane, Summoning
Standard Action Ranged 10
Effect: You summon one creature of up to the Wizard's Level or twenty-two, whichever is lower. This creature lasts until the end of the Encounter or until it is slain. Alternatively, you summon 1d3+1 Minions of the same level. All creatures summoned by this spell can each take a single Move action each turn without the Wizard giving up his own.
Summon Monster VI
Wizard Attack 23
You conjure a strong creature to destroy your enemies.
Encounter * Arcane, Summoning
Standard Action Ranged 15
Effect: You summon one creature of up to the Wizard's Level or twenty-six, whichever is lower. This creature lasts until the end of the Encounter or until it is slain. This creature may take a single Standard action each round without the Wizard giving up his own. Alternatively, you summon 1d3+1 Minions of the same level. These can each take a single Move action each turn without the Wizard giving up his own.
Summon Monster VII
Wizard Attack 27
You conjure several strong creatures to destroy your enemies.
Encounter * Arcane, Summoning
Standard Action Ranged 15
Effect: You summon one creature of up to the Wizard's Level or thirty, whichever is lower. This creature lasts until the end of the Encounter or until it is slain. Alternatively, you summon 1d4+1 Minions of the same level. All creatures summoned by this spell can each take a single Standard action each turn without the Wizard giving up his own.
[/spoiler]