I've read Caleb's guide on how to make a custom side, but I don't know if I should get every unit .TGA, .MSH, etc. and put it my custom side's folder. Example:

imp_inf_officer
imp_inf_officer_gray
imp_inf_officer_snow

Do I put all three variants of the officer in the custom side folder or just the regular variant?

Usually the one .tga file you need, because your unit's odf reads the unit's model. And this model actually reads the .tga file you'll need. So you can put them into your side's folder. But you are also able to change that in your odf's, or in your .lua.

So if I'm making a snow map, put the imp_inf_officer_snow .tga in the folder?

If you want. The snow officers is (by the way) the officer you have if you create a new map. The one with the gray helmet, the green uniform. He is also in your map. But if you want an other model you need to open the odf of him and change the .msh and geometry name.

Alright. Thanks for the help.