Yes, as Skull said, I will give you the option to replace the current in game guards or keep the guards and just have the skins. With a very basic level of coding you can customize many portions of the game. For example, you could make it that the In Game Soldiers stay as guards on the islands, but the new ones function as marines. That is, of course, up to you.
Sailor_Al --> Yes, they are compatible with the build. Well, I shouldn't say that. To be honest I don't know. I don't really play the game anymore so I don't know what the build all entails. But, my preliminary testing seems to work fine. I'll have to defer to Skull on whether or not it is compatible. I have not made the clothes "purchasable" from the tailor. If someone could figure out how to do that and tell me, I'll add that feature to the French Pack.
Regarding the Interface photos. In the Spanish Skin Pack I only made them for the Admiral, Captain, Lieutenant, and Infantry Officer. I did not make portraits for the regular soldiers. This is probably how I will do it for all of the packs. My thinking is that I assume most players wouldn't use them and it takes up space for face_id's. Also, making portraits is probably the simplest graphical change for the game. If someone wants them they can make them quickly themselves. Sorry if that is an inconvenience for anyone.
Best mates,
JMV