The maximum number of units in a unit_prod.txt is 256. The XL mod has more than 256 and any extra units you put on the end show up but cannot be built. You can add agents and ships past the 256 limit but any soldiers you add aren't going to work.

You will have to overwrite (or replace) one of the other units to be able to recruit your new troops.

As for the graphics, you might've made a mistake somewhere when you were in the folder "Textures" - make sure the new unit has a proper text file in the "ActionsPage" folder.

Again, any questions, just ask.