Will there be a tech tree or is it replaced by founding fathers?

No tech tree. And it's not precisely replaced by the Founding Fathers, either - there is no military advancement as such in Colonization.

I see. Well I guess the FF's replace great people and have the effects of Wonders.

FF's replace wonders, not GP's.

If anything, books which educate untrained colonists (produced by schools) replace the tech tree.