A greenfield deployment, or building a network from scratch, is perhaps the easiest in terms of co-exist an SDN OpenFlow network with traditional network. The only interaction between your SDN network to traditional network equipment will almost only be at the edge, which in today's world, almost always uses BGP as the exchange protocol. Your external facing node will be using BGP, such as what we have seen with the Ryu BGP library, while internally you are free to use any type of connectivity and topology you wish.
If you already have a way to isolate your datacenter network with this approach, in the case of a greenfield deployment you can use the same method. If you use a bottom up approach, you can start by calculating the amount of compute resource you need, tally it up to the number of racks, and determine the number of top of rack switches as a starting point. Then calculate the number of over subscription, if any, at each of the aggregation layers, leaves, spines, cores, and such. In a way, this is no different than planning out a traditional network. The only variables in this case would be controller placement and orchestration.
For the controller placement, there have been many studies and use cases published. But the final answer typically resides at the answer to the questions, "How many nodes do you want each controller to manage?" and "How do you want each controller to communicate with each other?" The first question deals with the SLA-level of how big of a failure domain do you feel comfortable with (we will look at controller redundancy later on). The second question typically deals with the type of coordination between the controllers. The coordination can happen externally or simply tied in a statistics feedback loop. , the leaf controller can simply know that for certain destination x in and ECMP format, it can be reached through ports 1 - 4 of switch 1 as long the utilization on those 4 ports are below 70 percent. Using the OpenFlow port statistics, the controller can easily be alerted if those ports are above the utilization threshold and . There are also commercial vendors who can provide this orchestration for open source OpenFlow controllers as well.