Orochimaru released the four previous Hokages from the Shinigami's stomach. Were there only four souls sealed inside its stomach? Or could the user choose to release only specific souls?
The Shiki Fuujin was invented by Minato, and taught to Hiruzen. That was said during Hiruzen's battle with Orochimaru.
It was only ever used two times, once when sealing the Kyuubi (which caused himself to be sealed in the Shinigami), and once against Orochimaru by Hiruzen (which caused Hashirama, Tobirama, Hiruzen and Orochimaru's hands to be sealed).
Because of that, no other souls were sealed in the Shinigami's stomach.
True enough, the author later contradicted what he said at the beginning of the series, and said the the Shiki Fuujin was invented by the Uzumaki clan, and taught to Minato (probably by Kushina).
In that case, it may be possible to only unseal the specific souls you want to.