NPC Options
Depending on what type of NPC you have purchased or can request due to a rank purchase, there are customization options available. For each specific NPC type, refer to their store page in the Misc. category to see what options are available.
Skins
Some NPC types allow you to specify their skin. In this case, you have 3 options:
Note that currently, it is not possible for NPCs to use the Alex model, so if you use an Alex-formatted skin, the texture on the arms will be slightly distorted.
Child-Sized Model
Trainers only (no other NPC type) can use the child-sized model. However, custom skins are not possible at all for this model type so you may only use the skins available in default Pixelmon. Some additional child-sized skins can be seen by the trainers at /warp Mall, you can include a screenshot to select one of those.