2
1) Most XOOPS modules work fine in XOOPS 2.5.5, but you need to test the modules that you use to make sure. Just install XOOPS 2.5.5 as a test installation, and test the modules there.
The biggest challenge is if you're also changing the PHP version, as some modules might not work properly, but again , this is a very small number of them.
2) There are several modules that have been updated to the new XOOPS 2.5.5 Admin GUI. See
this News, and see if they are on the list there.
So again, the best way is to test them.