WOW...
You only had it a day or too and you already no how to PMI characters thats impressive.

Unfortunately shooting them out of a gun is not enough to make them "Alive".
Heres a quick tut ill throw together for you
Summary: The only way too make working AI is too spawn them in vehicles yes i know thats weird but trust me its basic knowledge of modders

this is a tut on how to do this.
- Download HHT (its a dependencys swaping program similar to HMT but its not HMT its HHT {that confused me the first time})
- Open the map you want to make the AI work in, in HHT
Now for the tricky part
- The tags in HHT will be listed similarly to HMT find the vehicles tag.
- Pick the vehicle you want them to spawn in lets say the warthog
- Now on the column on the right a bunch of tags names will be listed. These tags represent all of the parts of the warthog tag you selected like model (mod2) and Weapons (weap).
- In the list scroll down to the SECOND ACTV tag (actor variant).
- Select it and click on the drop down menu underneath it and select the ACTV tag that corrosponds to the biped you want to spawn in the seat.
NOTE: you may not find an actor variant tag for the elites you imported into the map you will have to go and PMI the ACTV tags from another map into this map with the bipeds.
- Once you select the ACTV tag for the biped you want to place in this warthog hit swap and then hit save changes on the left side of HHT.
- Now open the map in halo you should have elites in the warthogs somewhere but be warned they drive like drunks (lol)
If you need a better tut or dont understand ill help you out some more but for now i got to go sorry the tutorials a little muddy

but i hope its ok seeya!