First of all, you don't need to and probably shouldn't change the race. In most cases, you also don't need to change the model. You were getting crashes because you entered a model that didn't exist.
What you want to do is change your appearance line so it looks like this:
modela texa
P_BastilaBA XYZ_Bast
And then take a copy of P_BastilaBA01, edit it however you want, and rename it to XYZ_Bast01. Substitute XYZ for whatever prefix you want, anything that guarantees it's a unique file name, not used by the game or other mods (e.g. I would go with something like JC_Bast or if I had to do several different ones, JC_BastA, JC_BastB, etc).
Appearance.2da has essentially two options. If the texture cell is empty (****) then the model will only render any textures that are on the model. When a string is input (e.g. P_BastilaBA) it is used as a prefix for the texture. This gets added to the two-digit texture variation on the item (default 01) giving the final texture name (e.g. P_BastilaBA01).