Well for a wtw 1.0 submod I think it would be easier to just ask the team leader HannibalExMachina for permission and credit those who worked on it. That said I did message others on the WTW team and they've given me permission to use their work.
Mammoths will most likely be added at some point, I'll ask AdmiralThrawn if I can use his mammoth unit from the AOPK mod.
What do you mean by horse costumes? As in the barded horse?