This is not very specific.. do I need to just ask about specific models on what terms they are held in ZK so I know if I may alter and reuse them?Copyright
Zero-K uses artwork (models, textures) donated from various sources under open-source licenses, particularly the GNU GPL, Public Domain and the various Creative Commons licenses.
These may be copied and edited and used in derivative works according to the terms of their respective licenses.
edit: yes, it seems like if one is interested they need to speak to a ZK dev about specifics?