Following the collapse of the Soviet Union, a few of the eleven of the former republics that were incorporated into the Russian Federation overprinted existing USSR stamps and issued their own stamps as a means of trying to validate independence from Russia. These became known as the "break-away republics", and were retained in the federation. The stamps were never recognized by the international community.
I think that would be fine.
re: Chechen Republic
Following the collapse of the Soviet Union, a few of the eleven of the former republics that were incorporated into the Russian Federation overprinted existing USSR stamps and issued their own stamps as a means of trying to validate independence from Russia. These became known as the "break-away republics", and were retained in the federation. The stamps were never recognized by the international community.
re: Chechen Republic
I think that would be fine.