Answer is always a big YES if you ask us. :-)
If these components expose API which you can consume using java then I don't see why you can not but of course devil is in details. Better to do a POC first.
This is a fairly nebulous question and would really depend on your requirements, but I'll toss in my two cents.
First, to your comparison - Trac is a custom app written in Python.
Second, you have to think about what a Portal platform and specifically what JSR168 and JSR286 buy you. The portlet JSRs define a framework for building modular components, which doesn't sound too far from what you need - but, coordination between components is limited - though much better with JSR286. The portlet specs also talk a lot about security and JBoss Portal is packaged w/the JBoss Identity API which you could leverage.
Though, on the other hand, developing in a Portal framework can also be restrictive and provide additional features you may not need. For example, does a theme framework make sense for your project tracking/mgmt software?
Honestly - it sounds like a custom app to me; if I had my choice I'd take a look at http://www.seamframework.org/.