From a pure RDBMS standpoint you are asking for views and a table for some "meta data" on your clients. Views are just a stored SQL statement such as "select x.a1, y.a2 from x, y where x.id = y.id" that you can name something like "cust1_feed4". As column needs change, just change the view for the client.

Assuming you have multiple feeds per client, the meta data on each client would be stored in a "client" table that has a structure like this.

clientID, feedID, viewName,FormatFlag, ...  

You should hire a DBA or really dig into some basic relational principles. Too often, developers with little or no experience in relational data base management architect themselves into a corner by not understanding key concepts like rationalization.

A good DBA would have a structure outlined for you in half a day or less complete with optimized indexes.