There are similarities as both RUP and Agiles are trying to apply best practices; For example iterative development.

As you said RUP could be customized and is not some thing on the stone, So we can have
an agile customized RUP that could be very similare to XP for example. But what I underestad, the major diffrence between RUP(or RUP like) and agiles is that RUP has an Elaboration phase but agiles are applying this requirement by refactorying during all phases of project(Correct me if not)(Ofcourse RUP let you to refactor as well).

About the large  scale and enterprise,I think none is the root reason to see if you need the elaboration phase or not but they are effecting parameters on your decision; If you see the risk of refactorying architecture is not high(By considreing all parameters like large scale, type of project, your company experiences on that type, You architects experiences,...) in your project then it is ok to ignore elaboration but if you can't have any estimation on the risk or you have but you see it is high then better to have the elaboration. Ofcouse the size of project is one the most important parameters on this decision.