Coste's 7-string was in standard tuning with an added bass, usually tuned to D. The Russian 7-string is tuned to an open G major chord, DGBdgbd' from low to high. Coste's music wasn't composed for the Russian tuning.
Check the Editions Orphee site for more info and some pieces composed for Russian 7-string. Volumes 9-10 of his Russian Collection are in the original tuning. The earlier volumes were arranged for 6-string guitar in standard tuning.
In terms of obtaining original sheet music, there are some in the RiBS collection by Sarenko, Alexeev, and probably others I'm forgetting now. The Boije collection also contains some Sarenko and Sychra. The etudes by the latter are excellent, but I don't know if there is a legal online source.
Remember Anthony Weller, please help. Contact myself or Aaron Green for details.