Building a network is a difficult task, especially if you are going to mix equipment from different vendors. Normally this is not done due to issues that may arise in your network. However, this is not to say that it cannot be done.

You need to make sure that you have equipment aimed at the correct level - try to avoid using provider edge equipment for customer edge implementations. Also check that the equipment have the same feature sets - eg. MPLS, RSVP and so on.

I would also consider the cost for staff to become familiar with all of the equipment.

regards, Mark