my quation is - is it possible to use my existing cisco cat 3650 in the Access-Lyr and use the Nexus 7k in the Aggregation Lyr?
Yes, there is no issue with connecting 3650 to 7ks.
Also What is the deffence using Nexus 2000 and or Cat switches as the access lyr lyr devices when using Nexus 7k in the aggr lyr?
The difference is that the 2ks need a parent switch(s) (5k, 6k, 7k, etc..) to operate and manage and they will be installed as blades to the parent switch. On the other hand, the 3560 is a stand alone switch and will managed on its own. If you are trying to build a new data center, you want to use the 2ks as they are designed for data centers.