Hybrid routing Framework like ZRP, MZRP which address Overlay Space, Clusters, periodic flooding , broadcasting are suitable mechanism to solve scalability issue in MANET.

use of Hybrid/Hierarchical routing protocols (ZRP,MZRP,CGSR) and other protocols  such as FSR is preferable as compare to other protocols. Scalability 

Using Hierarchical structure in heterogeneous network will be the best solution for ad hoc scalability problem. I have a paper in this subject that will be publish in IEEE in a few weeks. its name is "DSDV-Het: a new scalable routing protocol for large heterogeneous ad hoc network"

I simulate my approach in NS2 and achieved good results

I think you should go for Distributed Hash Table(DHT) in MANETs as well as Clustering......