I have never seen a texture pack using the format mobnameAlt#.png But I do know that the MCpatcher (an HD patcher, etc) supports alternate mob skins in texture packs using the format mobname#.png
so if its a 16x16 pack and i just make the alt. colors and name them Zombie1.png, Zombie2.png, then use the mcpatcher it should work?
I usually use regular old default but if i find a texture pack with something I like thats 16x16 i'll take it out and put it in the default. I end up with a really pleasing frankenstein texture pack