Well, I can say that the .tpa, .tpb, and .tpc won't work out... The reason for this is because of how KotOR 1 and 2 do their textures, as well as the formats they support. I will endeavour to explain below: The Texture Formats:
TPC -> Native format that the game's original textures come in. This format is a modified form of the DDS file format, and the game's files make use of DXT1 (if there is no Alpha Layer/channel in the image) or DXT5 (if there is an Alpha Layer/channel), resulting in a 1:8 or 1:4 (approximate) file-size ratio. In addition, this file format supports mipmaps, which are basically a series of bytes that allow varying levels of detail the closer the camera is to the texture being rendered, as well as including the textures special information (what you'd typically find in a .txi file) at the end of the file itself.
DDS -> Essentially, this is the same as the TPC file format, though again, the TPC is slightly modified from the DDS so there is some variance. Both seem to support mipmaps, though I know for a fact that the DDS doesn't support the .txi information being embedded into the file.
TGA -> Undoubtedly, this is the most common format used by mods. This is due mostly to the ease of access, as we don't have a TGA-to-TPC converter, and the DDS file format requires a special plugin for both Photoshop and Gimp (and the PS one can be really slow; I can't speak for the Gimp one...). This format is undoubtedly the biggest of the files, since most users will use a 32-bit version, even if there is no need for the Alpha Layer in the texture. Also, there have been varying levels of success and no confirmed reports as to the use of the RLE Compression when saving the textures themselves.
The game's Precedence Order, or Hierarchy if you prefer, isn't clear with textures. We know that the game will use a texture in the Override folder, but we do not currently know if it will use a TGA before a DDS, or a DDS before a TPC, for example... The Texture Storage:
The games support three texture options: Low, Medium, and High. These are controlled by the texpacks.2da, which is pictured below:
First off, no; I've already tried it, and we can't seem to add our own options. The game went back to High...
The first column is the row number, the second is a label for description, the third the name of the texture .erf to use, the fourth the GUI .erf to use, the fifth is the TLK reference to the text used in the Graphics Options in-game, the sixth and seventh have to do with the Graphics Engine, I'd imagine...
The Texture Packs map onto the tpa.erf, tpb.erf, and tpc.erf packages in the Texture Packs folder. Each .erf package contains the same textures, though the number of pixels (and thus the quality and filesize) varies between each, and all are in the .tpc file format.
Most modders extract the textures from the tpa.erf, since the letter denotes the quality level, in reverse order; tpa.erf maps to the High setting, tpb.erf to the Medium setting, and tpc.erf to the Low setting in-game.
As to what the modders can do, it is entirely possible that they could downsize their textures to fit a lower quality, should they choose to do so. In this day and age, most just assume that people have good computers that can handle games like Skyrim (and thus can handle a few HD textures in KotOR...), which admittedly can be argued as a fair assumption...
P.S.: Of note is the fact that the .tpc and .dds files are better in both file-size and performance, and so are highly recommended to use (hence the TGA-to-DDS Converter here) both for download size and load-time in-game...