Hello Sharma
1. You can replicate from MDG to ECC if MDG is not on top of ECC i.e installed as hub.
2. If MDG - Material / Supplier / Vendor is installed on top of ECC then data is directly updated in ECC tables. In that case no need to implement replication model.
3. For MDG- Finance, since its a flex model, you have to configure replication model to replicate the data from MDG - F to ECC. You can use IDOC / SOA or file transfer method.
Thanks
Kiran