To answer queries over a federation of multiple data sources, a preliminary task is source selection; i.e.,identify the federation members that can answer each part of the query and decompose the query into subqueries assigned to each federation member. Existing source selection approaches for federations of RDF data sources have been developed based on the assumption that the federation member are SPARQL endpoints. This paper presents an analytical study that investigates whether these approaches are still effective in the context of federations that are heterogeneous in terms of the types of data access interface. In particular, we identify what information about the data of the federation members is required by the approaches and analyze the possibilities and the effort of obtaining this information via the different types of data access interfaces. We find that almost all existing source selection approaches can be adopted for heterogeneous federations but obtaining the required information may not be practical