For the first question, I'll re-quote my last two paragraphs:
As to your next question, you will need to:
1. Open heads.2da
2. Find a twi'lek head (take note of the gender)
3. Check the row's value for the head column (for example, we're using row 61 and 62, female and male respectively)
4. Make a new row in heads.2da, and make sure you change the row number
5. Give the head entry the name of your texture/model. The head column is the model and neutral/LS texture
6. Add in names for DS transitions if you want. (Immediately after the head column, we have most evil and it progresses to least evil)
NOTE: KEEP THE HEADS.2DA OPEN!
7. In KotOR Tool, go to BIFs->Models.bif->Aurora Models, find, and extract the .mdl file for a twi'lek head (twilek_f and twilek_m for example)
8. Go to the Aurora Model Extensions branch and extract the .mdx files for a twi'lek head (again, twilek_f and twilek_m for example)
9. Rename both the .mdl and .mdx to whatever you put in the head column in your new row in heads.2da
10. Go to ERFs->Texture Packs->swpc_tex_pac_a.erf->T and extract twilek_f01 or twilek_m02. SAVE THEM AS TGA FILES!!!
11. Rename the textures to whatever you put in the head column in your new row in heads.2da, BUT KEEP THE NUMBERS!!!
Note: Change the number on the male twi'lek texture, if your head is male, to 01.
12. Click somewhere else in the heads.2da file and then save it to your override folder.
13. Put the row number of your new row in heads.2da into the normalhead column for your new row in appearance.2da.
14. Lastly, click somewhere else in the appearance.2da file and then save it to your override folder.
That should do what you need, but if the head shows up white or something, then you'll need to hex-edit the model, which I can walk you through if necessary.