I had two question about Rod of Giant Summoning:
1. If a creature summoned normally comes with a weapon, e.g. a cyclops using Summon Nature's Ally V, does that weapon get resized. The polymorph rules and Enlarge X spell specifically says this happen, but those don't cover this case.
2. Can a creature be targeted by a size increasing effect if summoned using the rod? For example, could a druid summon a Giant Velociraptor and then cast Animal Growth on it? It seems to me that the Giant template isn't a size increasing effect, but rather something that changes the base size of the creature. Such a summon creature should still be able to be enlarged, such like a creature that is natively Giant or an animal companion whose 4th/7th level advance has increased its size.
I'd probably allow it. There's no discernible balance issues. Although I understand the notion that a template on the creature may not affect its equipment. Consult your GM. If this is for PFS, consult your GM and expect table variation (barring any further evidence).
Multiple magical effects that increase size do not stack.
Both the metamagic rod and the spell itself would count as "multiple magical effects that increase size".