Yes you can use Acegi instead of Seam security in a Seam application (I have done it - but now I'm using Seam security).
I used Acegi because Seam security didn't exist in Seam when I started to use Seam.
I will recommend Seam security because it's a lot easier to use in a Seam application :-)
BTW: Why do you think Acegi is more powerful? You can use JBoss Rules (can be very complex) in Seam security.
Is there a specific case what Acegi can do and Drools rules can't?
I just had a superficial look into this topic for Seam. Acegi is complex ... for many of my colleagues an enigma. But we've been able to handle very requirement regarding Authentication/Authorization. What I like on Acegi is that the application code can be written at first without any tags or security considerations like specific roles etc.. Later on you can configure the usage of it and implement required authorization classes.
To hear that Seam Security Layer and JBoss rules are similar flexible to Acegi sound good. Currently we have some framework classes developed based on Acegi for integrating an application in our infrastructure.