since you have 1wan and 1lan system , you don’t need to use bridge.
Try use normal setup of zeroshell where eth00 is for lan and eth01 is for wan/dsl/pppoe etc.
You can easily do qos on normal setup , where eth00 will became outgoing interface for packets coming from wan/internet to lan , here you can apply qos and limit the bandwidth. The eth01/ppp will be outgoing interface for traffic coming from lan to wan/internet , and you can apply the qao here also.
You need to substitute eth01 with ppp0 if you use ppp/pppoe , then you don’t need to apply qos on eth01 only on ppp0.
The mechanism for creating class and classifying the traffic is same as described in qos article.
Bridge can be extremly useful , if you have another router or server which is either incapable of doing qos or runs a cache server(which makes little difficult to do a qos). Bridge has other uses also.