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)

c) adjust/config the tnsnames.ora for 6i

And that´s it !