What AIX version I need to run a software in a R6000 model 6015/6020 in a IBM P5??? Im implementing a Hot SIte. Is this possible or I need identical systems hardware and software?

There are no such models of rs6000, p5 or otherwise.
Do you mean p615 and p620?
The p615 is a power4 machine and the p620 is some earlier /older processor model.
Minimum AIX for the p615 is about 5.1 ML03
Minimum AIX for the p620 is 4.3.3 ML08
Maximum AIX for the p615 is 6.1
Maximum AIX for the p620 is 5.3
If both systems can run the level you choose then you should be able to backup one and restore it to the other.
HTH