We have installed and customized (Apex, VF and triggers) app exchange product , Custom objects from Managed package is not counted against the Salesforce limitations but Apex classes and triggers, is it expected behavior?
As per the document on Governor limits
Certified managed packages (managed packages that have passed the security review for AppExchange) get their own set of limits for most per-transaction limits.
All per-transaction limits count separately for certified managed packages except for:
The total heap size
The maximum CPU time
The maximum transaction execution time
The maximum number of unique namespaces
These limits count for the entire transaction, regardless of how many certified managed packages are running in the same transaction.
Also, if you install a package from AppExchange that isn’t created by a Salesforce ISV Partner and isn’t certified, the code from that package doesn’t have its own separate governor limits. Any resources it uses count against the total governor limits for your org. Cumulative resource messages and warning emails are also generated based on managed package namespaces.
Do check if the managed package that you have installed is certified or not.