The terrain to Madrid is very rough and Spanish barracks could easily appear behind your advance. Spain can also keep a heavy supply of resources from the colonies. Only consider this if you can get a firm commitment from Portugal that he will also attack. Needless to say there will be many nations interesting in attacking Spain's colonial resources. An attack on Spain by France is actually worse for Spain that Vice Versa, as this effectively stops all his land based trade.
Spanish troops are generally poorer quality than France's and more expensive.
As France, it is almost impossible to engage in piracy without the target finding out. France has many barracks easily reached and will probably have a spy from one nation or another in each. The Moroccan navy is slightly larger than yours as is the Russian but these are of inferior quality. If you have no need for your ships in Europe, use them to ferry troops and commanders to the colonies. 1 commander and 1 brigade conquers as much territory as a brigade of KT troops but at around the cost.