Great discussion!!
As a commercial civ on Diety, the knowledge of Alphabet, the most expensive first tier Ancient Era Tech, is a tremendous trading advantage, but only if you can make contact with the other civs first. Couple that with knowledge of the Wheel, and the Spanish will be able to obtain nearly any early trade they desire. I'll first check the F10 - Space Race function to see how many other commercial civs there are. If there are several (which is likely), then I know that the clock is ticking. I want warriors out scouting ASAP!!
This makes the hill location to the NE even more appealing. If I move my Settler NE, then I gain visibility over many more squares, I have a more defensible Capital city location and I'm still adjacent to one of the known forest squares. Assuming nothing exciting is uncovered, I'll found Madrid on the hill, and start irrigating the Floodplains just South of Madrid. (May require a 2nd worker move if I choose to scout first.)
Now, to speed my first warrior, I'd start my citizen in the Forest for 3 turns, then move him to the Floodplains. I'll have lost out on 3 food (which is one turn once the Floodplains are irrigated) and 3 commerce. However, I'll build a warrior 6 turns earlier. I'm betting that the extra 6 turns of scouting may mean the difference in making critical early trades with nearby civs. I might repeat, or switch into growth mode once the irrigation is finished.
All the nearby plains will require improvements; this may necessitate an early 2nd worker. These plains spaces are very flexible; typically I'd irrigate them to get the extra food and maintain the production. Because of all the floodplains, I plan to mine some, giving me the same food/shield/commerce ratings as Hills is Despotism, but with half the worker movement cost. Having some plains be irrigated and some mined means a lot of MicroManaging; skills at MM will be needed for this terrain.
I'm planning to research Writing at minimum from the start; I may increase my research as I learn more about the game. With the Spanish starting Techs and the right contacts I should be able to trade for most or all of the first tier techs. If there are no nearby commercial civs, I might actually get Writing first, which should gain me a number of 2nd tier techs, also. I don't plan on building any early wonders; I'd like to make it into the Middle Ages!!
I haven't played as the Spanish, and I'm not sure of the best use for their UU. Resource pillaging sounds quite good; perhaps place several Conquistadors several spaces into enemy territory to pillage resources, cut supply and access roads, harrass workers, defenders, etc; while the main attack force is taking border towns. If the conquistadors get beaten down, they can quickly retreat to friendly territory. I doubt that much of the map will be available to explore by the time they get around, but I could be wrong.
Those are most of my thoughts and plans going in.