In all honesty initial population isn't too terribly important. Most Latin American nations received large amounts of Southern European immigration during the later 19th century (some more than others obviously) and its fair to assume that the immigration would have started even earlier, if said nations were both stable and prosperous.
Gran Columbia if unified and stable, has alot going for it. Energy wise it has significant coal reserves in Columbia, major oil deposits on both Venezuela and Columbia, as well as enormous hydroelectric potential. Mineral wise both Venezuela and Columbia have large amounts gold and gems, Venezuela has major deposits of Iron and Aluminum, not to mention smaller deposits of scattered about it region. Geographically said nation would have access to both the Atlantic and Pacific, and would most likely control the Panama Canal. Columbia and Ecuador both possess exceedingly fertile soil, and productive fisheries.
Considering this, Gran Columbia has a great shot, if stable, of becoming a great power by the 20th century.