to say that this will work or not you should paste mangle configuration...
There is catch with q simple & q tree in one box - yes that possible, but only in one (maybe there are more possibilites, but let assume that normal MT user don't want to know all the details regarding traffic flow, IMQ's, etc) specific case - you will take the _rest_ of that queue simple left to you.
I mean that you need to look at the traffic flow (in manual) diagram and prefectly set up chains in mangle and interfaces in queuetree to catch&queue _any_ packets.
Then there is another problem regarding to PCQ - yes it will make some "fair queueing" but only when your overbooking is low (you sell about what you have)
In situation (for example) where you have 2Mbps and 50 clients with 256Kbits - this will not work correctly in most cases
But if you have only 20-25 clients with 256Kbits - it shold work fine on 2Megabits
I suggest you to make everything in Queue Tree, use SFQ instead od PCQ, try to guarantee traffic to provide "forced" fairness
if you want i can help you little with this setup,
you can look into this thread and check my example setup of 2Mbits BM