I doubt that at the very early stage of conflict they even had any uniform.
So irregular Bulgarian might help. As I recall what my grandfather (being a Rumelian himself) has told me he could recall only bulgarian uniforms, but:
I found this link:
http://warrelics.eu/forum/imperial-russian-white-army-pre-1920-history-uniforms/antique-photo-unknown-russian-uniform-constantinople-26713/which sounds that it is correct. The uniform looks like Bulgarian and not imperial russian.
Consequently I suggest you the following:
early stage: irregular bulgarians mixed with armed civilians (for example Plovdiv rebellion)
middle stage: similar to the uniform as above link (for example Serbo-bulgarian war)
Later stage: regular bulgarian uniform of balkan wars era due to the official unification with Bulgaria.
Hope to helped you.
Although I have roots in Eastern Rumelia, I am not an authority and , thus, I dare not post any link abt history etc.
Try searching "Eastern Rumelia militia" and you might get some more interesting results.