Also see the feats:
Imbued Summoning, PHB2 p92 (summoned critters arrive w/beneficial spells)
Cloudy Conjuration, CompMage p40 (summoned critters accompanied by choking cloud)
And consider these options as well:
1) Summoner Domain, CompDiv p141 (adds +2Cl to summon spells)
2) Domain Wizard ACF, CompChamp p52, lets a Wizard exchange a Wiz bonus feat for access to a Domain's granted power. If the PC has any Cleric levels, he gets full access to the Domain. As a gestalt Wiz/Cleric, that means you'd have full access to 3 Domains...and I think the above mentioned Summoner Domain would be an excellent choice.
Combined with the reserve feats I mentioned before, you'd be adding +4CL to your summonings.
3) UA, which I don't have on hand, has ACFs for conjurors that boost your summoned critters' power.