Since you haven't received an answer, I'll provide my thoughts, hopefully somewhat helpful:

There are many ways to set this up, presumably you are following instructions somewhere and they rely on a reference architecture which specifies the host interfaces and switch ports. If you are not following instructions, I think that's a mistake and you should. For CentOS, the Newton release instructions call for two interfaces per host: http://docs.openstack.org/newton/install-guide-rdo/environment-networking.html. In the network layout there, your existing network is the red one.

How many switches you have is not as important as the VLANs and routing that you setup on those switches/routers. You could do this with one switch if you had enough ports and could set the ports to be in the management or provider network.

Sorry, not familiar with fuel. Again, I know I'm not directly answering your question but since you have no answers I thought this may help you think through it a little.

Sincerely, David