If you cast a Dazing Summon Monster I does the creature have a chance to Daze someone whenever it deals damage?
If this doesn't work for A Summon Spell, what about Force spells such as Spiritual Weapon?
You can modify a spell to daze a creature damaged by the spell. When a creature takes damage from this spell, they become dazed for a number of rounds equal to the original level of the spell. If the spell allows a saving throw, a successful save negates the daze effect. If the spell does not allow a save, the target can make a Will save to negate the daze effect. If the spell effect also causes the creature to become dazed, the duration of this metamagic effect is added to the duration of the spell.
Spells that do not inflict damage do not benefit from this feat.
A summon monster spell doesn't do any damage, any more than a bless spell does. It just summons a creature, and the creature does damage. So a Dazing Summon Monster I doesn't do anything useful.
There's nothing inherent in force spells that would prevent it from working with them. Keep in mind the italicized text, though - they get to save against the dazing no matter what, at the usual DC for that spell, even if the spell doesn't normally allow a save. So a Dazing Spiritual Weapon would have a chance to daze on each attack, and require a save on each attack that successfully did damage. One of the better spells for attaching it to, actually.
Okay how about summon ancestral guardian. It's a conjuration (summon) effect, but they are treated as spiritual weapons except damage type and movement. So is it the spell doing the damage and benefiting from dazing spell since the weapons strike as spells? Or no, because clearly you are summoning actual ancestors, and the creatures not the spell deal damage?
The ancestors thus summoned aren't treated as Spiritual Weapons. They are sentient. Their attack and damage are on par with Spiritual Weapons.
As such, Summon Ancestral Guardian doesn't benefit from Dazing metagamics.
If you want a rule of thumb, consider that if the spell wouldn't break Sanctuary, if cannot benefit from Dazing metamagics.
Attacks: Some spell descriptions refer to attacking. All offensive combat actions, even those that don't damage opponents, are considered attacks. Attempts to channel energy count as attacks if it would harm any creatures in the area. All spells that opponents resist with saving throws, that deal damage, or that otherwise harm or hamper subjects are attacks. Spells that summon monsters or other allies are not attacks because the spells themselves don't harm anyone.