There is not really a minimum Joomla version as we still offer old RO CSVI/CSV Improved versions for download. So you could even be running Joomla 1.5 and download a version that runs on it.
As for J2Store I have no idea which version was used when we first released support for it but RO CSVI came out on October 9th, 2017 with J2Store support, so whichever version was current at the time.
However, we do not provide support for old RO CSVI versions, support is given on the current release available, which is RO CSVI 7.20 at this time of writing which supports J2Store 3.x and Joomla 3.9.x
So I looked this up and the last version to run on Joomla 2.5 is RO CSVI 5 and J2Store support was introduced in RO CSVI 7.3.0. This alone is enough to know that RO CSVI will not run on Joomla 2.5 with J2Store support.
I am afraid there is no version that can be used in your case.