Hi,
Are you planning to install reports 6 on the same machine where you have reports 2.5. If thats the case i dont think it will work becuase the installer will get upgraded.
Yes, you can run (and install) Reports 6i & 2.5 in the same machine - I have both in my personal machine right now (Windows NT with 128 Mb). I did this way :
a) install Reports 2.5 in the original home (OraNT , in my case)
b) install Reports 6i in the original home (DEV6i, in my case)