Mangle, PCC and fastracking works fine as long you work cleanly.
Fastracking in Firewall comes after Mangle, so Mangle has more influence. BTW you can also mark tot fastrack traffic in Mangle.
Remember that once marked fasttracked some traffic will go the slow way to allow checks and changes. Secondly fasttracking and using a VPN on the router don't mix.
thanks for the info, I am marking some traffic to go a VPN so the above comment of connection-mark=no-mark works well as I don't want to fastrack the VPN connections