Sebastian contacted me with some questions about OpenACS and I told him to post here for better information than I could give him. The conversation hasn't quite gone in the direction I'd hoped, so let's see if I can steer things in the right direction.
What he needs to do, from the sound of it, is set up an ASP situation where each of the smaller firms has it's own subsite. I know that this works in theory, but I've also heard about various problems with subsites. So what I think he really needs to do is hear from someone who has attempted a similar setup and can report on what really works and what doesn't. Beyond the basic subsite functionality, it would also be helpful to know which packages actually work with subsites and which are subsite amnesiacs.
As far as modifying things goes - it's easy to modify the look of each subsite, but beyond that you get into Real Programming pretty quickly. Those of us who have been doing it for a while don't usually find it too difficult (we know where the skeletons are hiding :) but it does take new users a bit of time to get up to speed.