Tercios are available earlier than pike militia, though. It's faster to build the military academy (cheaper too) than to build a militia barracks (6 turns and 9600 florins by comparison), and that's assuming you already had the level 4 barracks built when the city was upgraded. There are no prerequisites to the military academy. The Spanish don't have any bows other than peasant archers, but they do have pavise crossbowmen who have long range missiles. They've done extremely well for me in the time prior to musketeers.