Myself and my architect are designing the architecture for one of our products and he suggested to go with Web Api as a Service Layer (because it's a light weight component).
Straight away I'm thinking how it can be used for non-http based clients and also for external clients. How can we provide the proxy information about our API (in case the API is having a complex type as parameter)?
Please advice whether we can use Web API as Service Layer?
Straight away I'm thinking how it can be used for non-http based clients
Well, it can't. The Web API works only on top of the HTTP protocol. If you need to use some other transport protocols such as UDP you might consider WCF instead.
How can we provide the proxy information about our API
RESTful services do not have the notion of proxy information. They should be documented well for non .NET clients. For .NET clients you could share the contracts (Request/Response Dto) between your server and client application. The client application could then reuse those Dtos which may act as what you call proxy and which is something that exists in the SOAP world but not in the REST world.