CiviCRM Community Forums (archive)

the non-alt version zips up the main civicrm folder, then packages it with the installer into a containing zip.
the alt version is a single zip.

the non-alt version may install easier in Joomla because the file size is smaller and it will decompress quicker during the extension installer's initial steps. that's why it was originally created -- to address some issues during installation.

earlier versions of Joomla had some poorly designed zip processing. with v2.5+, the installer is more efficient and it doesn't really make much difference what install package you use.

I would say it's *mostly* compatible with J3.1.5 -- meaning -- it seems to run fine (in my testing and use), but it hasn't necessarily gone through a thorough testing regimen in that version, so there may be some unknown issues.

there's no direct importer from that tool. but you can look into the native contact importer, and if you are able to export records as .csv files from the old system, you should be able to then import using that tool (under the contact menu)