Personally, I think "use the right tool for the job". This is the s9y forum site... and we are not using s9y to power it. That should be your first hint.
You can very easily skin each of them to be identical, as was done for s9y. I think we have a total of 4 different scripts using an identical design - s9y, phpBB, cowiki and whatever powers the spartacus page (I forgot).I too needed a forum, and also use phpBB. s9y is installed in my root domain, and phpBB in sub folder. No problems.