I have a few boxes of Conquistadores, but I don't want to use them against aztecs/peruvian. I'd like to learn how to go about creating the Spanish Tercio or whether the conquistadores can be combined with any other existing plastic sets.
You've picked probably the worst represented period in 1/72 plastic. For purely Spanish troops you only have the Revell and Caesar Conquistadores to choose from and each of those has nowhere near enough pikes and firearms to do a decent sized Tercio.
The cavalry in the sets are handy but again too few. The crossbows are good for skirmishers. To my mind there are too many sword and buckler men but that is likely due to the sets being intended for the doings in the Americas.
Tercio info is easy to get on the 'net along with diagrams giving numbers, etc.
The Conquistadores can be supplemented with Landsknechts (the Spanish fought beside them at Ravenna and La Motta for example) and cavalry from Dark Dream. Light horse are much trickier, Redbox Scurrers do at a pinch. It all depends on how 'accurate' you want to be.
I used the earlier and smaller 'colunella' formation using Conquistadores.
It is possible but takes a bit of effort and searching around.