I've seen the brilliant species mod that overwrites several of the original species but what I want to know is, would it be possible for those new species and perhaps more to be added in addition to the original set?
Yes its possible, requires additional server code.
Tonberry as a total noob to modding anything, can you point me in the right direction on how to start learning to mod for SWGemu specifically?
I would really just recommend going to our tools section and downloading TRE Explorer. This is the main tool for modding so just grab that and get familiar with it. Once you've opened up one of the tre archives just have a browse through them, learn which files are where etc.
Checking our wiki will also get you familiar with which file formats do what and how they are linked to each other. We do have a few tutorials but nothing specific or in depth.
Just diving in and referring to the wiki when needed will be the best way to learn. The absolute basics of modding is essentially just renaming files so once you've got a feel for the client it should become a lot clearer, and then the more advanced stuff will click into place.
One other thing which can help is to download a few existing mods and look through them, see where they go, what they do and which file formats they include. This will help you a lot in terms of learning the structure of things.