Well I'm far from being a good modder (or even a modder per se) but, does that model have a .xac file? this .xac file IIRC is the 3d model, that scoutatoi model should also have .dds with the texture, most of them have the name "name"_diffuse.dds.
Now I do have an idea of how you could replace the vanilla model with your scoutatoi model, but it probably won't work and should try it AT YOUR OWN RISK.
1.back up everything just in case
2.go to your dlc folder and copy only the .zip files
3.create a new folder in your desktop and place those copies there
4. Uncompress each dlc.zip (cuz I don't remember which one it is)
5.Once uncompressed, find the dlc folder which has an "interface" sub-folder and inside that has "byzantine_avatars.gfx"
6.MAKE A BACKUP OF THAT FILE just in case
7.put your scoutatoi .xac and .dds files (and other files if it applies) inside installation folder/gfx/models
8.open the "byzantine_avatars.gfx" with notepad
9.inside that file find the line "name = "byzantinegfx_HEAVY_INFANTRY" and below you'll see another line which is actorfile = "gfx\models\Byzantine_Heavy_Infantry.xac"
10.substitute that .xac file with the one you just put in your CKII folder so it should look like this: "gfx\models\scoutatoi_or_whatever.xac"
11.save the modified "byzantine_avatars.gfx" and put it in your interface folder
11.run the game (there is a chance that your game will crash, if it happens just delete the "byzantine_avatars.gfx" that you put in your iterface folder and all should be back to normal)
anyone with more modding experience than me is free to tell me these are not the correct steps....I'm just going by a lucky guess here since I have done a little modding to my medieval 2 and CKII games
Out of topic, I've been waiting for someone to make a thread like this because I would like to change and mix so many of the models to give each culture some flavor
Here are pics of some little personal modifications I've made for myself so you can see for yourself that you can modify the sprites to a certain extent:
Ugric units (mongol unit with norse attachments)
Baltic units (Russian units with norse attachments)
Hungarian,Croatian, Serbian and Bulgarian units (byzantine units with vanilla-western attachments)
unfortunately this is the most I've been able to mod without the game crashing or the units looking weird.
(of course the historical accuracy of my little modifications is far from 100% but I think it's better than having them with vanilla units)
And since this thread is about changing models and all, may I ask to those who have more modding experience than me, is there a way to...for example, give a russian heavy helmet to the byzantine heavy infantry or something along those lines?