Hello.
Not many people have animations beyond halo 1. There are a few h2 or even h3 animations floating around in the halo CE community here but more developed modders are the ones who normally have them.
reach grunts, elites, and brutes along with other assorted ports can be found here:
http://haloce3.com/ce32013/ It is under
Demo/Tags called
Shadowslayer’s Assorted Ports. Give Shadowslayer credit, they look great. I believe they are open sauce shaders and stock/cmt animations.
Halo 3 brutes exist in a few places but only among a few modders. Can't help you there.
Like the first answer, drone and elite animations ripped from h2 do exist but only a few people have it.
I think I saw some skirmishers somewhere. You might want to look outside of halomaps
Plasma launchers exist but I don't think any of them were ever finished/released.
I think a good look into google or even youtube releases will get you a skirmisher at least. I remember seeing some. They had a poor rig but were usable.