I was wondering what mods could be implemented in the beta as of now? For instance can we put new units into the game yet or buildings? I was thinking about doing that recently but haven't seen anything on here about that yet.
You can replace the current models, but you can't add new units or building just yet, not until we have server side modding capabilities. There is lots info on this subject in the Planetary Annihilation Mods Forum.
I recommend checking raevn's stickie'd modding thread's it's where i started, then just experiment. I found most planet biome mods can be viewed in the system editor even though you can't use it in game. It's great to just play and learn about the .json's configuration.
UI modding is fine at the moment, as it's a client-side only mod. Pretty much well any client side only modding works (changing textures and unit models and biome model's). Most of the current client side only mods are for different icon's and adding extra information to the HUD an UI.
Another great way to learn what can be done and how to do it is to look through raven's mod list in this thread or his mod manager. Then just download the mods that you want to find out more about and check out the code. If you have any questions on the mod just post them on that mod's thread and the modder will probably be happy to help. Also, if you want to figure out more about the basics of UI modding you can check out one of the really simple ones. (My profile pic fixer is probably the simplest one out there and is linked in my sig.)