I would like to know what is the better design for the database.  The application uses Visual Force pages to perform CRUD methods on a custom object and at least 8 related list objects.  Each record of the custom object has about 5-50 records in each of the related lists.  A majority of the time only a single custom object record and its related lists is queried.

1. Is it be better to create a single related list object with a field that specifies the related list type in lieu of the 8 related list objects?  In this case all the fields on each of the 8 related lists objects would be put in the single object.  However, many of these fields obviously would be null. 

2. Is there a performance difference between doing 8 queries and 1 query where the same number of records are returned in both cases?  In the case of the single query, the records would be filtered thru code to put them into appropriate lists and is the 8 query method more efficient?

3. Is there a performance difference when a query when 10 fields are returned in a query versus 100 fields are returned in a query?  The 100 field query would be filtered thru code to put them into appropriate lists.

4. Is there a storage taken up by fields that are null in an object?

Thanks