Enlarge Person does not give the Huge evolution, it just applies the effects of Enlarge Person. Reach effects are an inherent part so thos apply, and you get the ability score modifications from Enlarge person.
The same applies to Reduce Person. Your reach changes appropriately, and you take the ability score modifications from Reduce Person.
I mean Reduce Person has a 1 round casting time and a 1 minute/level duration, but if you can work around those then go for it.
Enlarge Person does not give the Huge evolution, it just applies the effects of Enlarge Person. Reach effects are an inherent part so thos apply, and you get the ability score modifications from Enlarge person.
The same applies to Reduce Person. Your reach changes appropriately, and you take the ability score modifications from Reduce Person.
Size bonuses do not stack, penalties do.
Casting Enlarge on a large eidolon will give the eidolon -2 dexterity and the penalties to hit and ac. It will not give the eidolon +2 to strength, as the eidolon already has a +8 size bonus applied to it's strength. Your eidolon does benefit from the increased reach and improved damage die on it's attacks.
Casting Reduce Person on a huge eidolon will give it -2 strength, +2 dexterity and it will apply the size penalties for a large creature to it's to hit and ac. Reach will be reduced and damage die for attacks will be reduced by one step.