Aergon's Greater Fireball

Does this scale with the fireball tree in anyway? Trying to determine is spreading flames from this is stronger or if fireball is stronger or if they stack together

Does this scale with the fireball tree in anyway?

No.

Trying to determine is spreading flames from this is stronger

Apparently Spreading Flames applied from the explosion “deals 100% more damage.”
Meanwhile, fully talented Spreading Flames from Fireball has 40% more damage and 200% Spreading Flames penetration.

if they stack together

They don’t.
I don’t actually play a Spreading Flames (SF) build, but if it works the same way as my Haste buff overwriting, then when you hit with Aergon explosion, Aergon SF will overwrite a previous Fireball SF. And vice versa, if you hit with a Fireball, Fireball SF will overwrite a previous Aergon SF.

Someone else answered your actual question but I just wanted to say…

Did a rune master build with Aergon’s Greater Fireball and it just sucked the higher I got up in level so I wound up ditching it in level 80-something.

-To make the very most of it, you often need to create a fire gate, rush through it… bam… this sort of thing was inadvertently designed to fail in encounters like Julra.

-Also, other rune master options can do tons more damage without all the wasted time in setting it up.

-I am also aware that I could be wrong so if you continue to make this a GREAT option in higher content, let me know!