Well for what i know so far u cant combine infability to a agility couse the infability is greater gold runes vs puple rune so gold has a higer rank if u see it that way

and when u move a rune on top of another rune a box pop out saying what rune will absobe the other one. Up to u to hit ok or cancel thats the lock for me so far

u can always just move one rune to the right side and hit combine all

NOTE: CAREFULL WHEN U DO THAT ALWAYS CHECK U HAVE THE RUNE THAT U WANT TO LVL UP THERE

so all the small ones give u the exp to it and u be ok