If you make a sprite of a command center on a UMS map, can you infest it?

and how about on a melee map?

Unit sprites are basically the same thing as units, unless you disable them or something. They are removed in Melee maps, however, all other sprites, such as weapon sprites and such, are not removed.

So in answer to your question, Yes, but it's not really a sprite at that point.

No, it will disappear on a melee map.