I seen this before and I havent confirm it yet but maybe some of you guys might know.

Summoner stats carry over to your summon. Meaning if you think garuda is stronger than ifrit you are right cause garuda stats is int and summoner will go for int cause its their primary stats. (Unverified)
Ifrit need str to increase their damage output (unverified)
Titan need vit to increase HP. (This one i tried i put on vit on my bonus stats and surely titan HP increase.) Verified