> Do I tell Installscape where DFII installation is already and then install Incisive ?
No, you can install anywhere you like. The installations are independent (-ish) of each other.
> Does Incisive have to be installed before Cadence DFII or can it be installed after the Cadence installation ?
No, you can install anywhere you like.
> Any special considerations for Centos ? Will Incisive work on Centos ?
It will work, but it is not a supported platform, thus if you bump into issues, you might have to solve them a bit creatively. Centos is quite similar to Redhat which would be your starting point.