if I install 2.7 instead, will my experiment still be absolutely valid
I wouldn’t think so. The 2.7 is a development branch, which means that many things would be sufficiently different from the stable 2.6 or the subsequent stable 3.0 versions, or even between minor versions within the 2.7 branch themselves.
populate the 2.6 DB with some sample data and attempt to migrate it to 3.6
Please note that the only supported and tested binary upgrade path is between major versions, e.g. to upgrade from 2.6 to 3.6, you would need to upgrade to: 2.6 -> 3.0 -> 3.2 -> 3.4 -> 3.6. Another method is to dump/restore your data using mongodump and mongorestore.
where can I get an .msi installer of 2.6?
All available binaries should be available under the link “All versions binaries”: With SSL (https://www.mongodb.org/dl/win32/x86_64-2008plus-ssl ) or without SSL (https://www.mongodb.org/dl/win32/x86_64 )
Credit: Internet (found the exact problem statement)