Q1, installing 9.1 and 9.6.1 in parallel:
In general you can always install n versions in parallel, no technical constraints. However, there are quite a few points to observe before doing so. Here are only those which I recall off the top of my head:
- Installing both versions on the same box is only possible on Unix/Linux, not under Windows.
- Installing both versions on the same Unix/Linux box only works if you can keep Java environments separate AND the DB clients are available in compatible versions for both installations.
- You can upgrade the 9.1 contents to 9.6.1, but then you can't run these 9.1 environments any longer (INFA software is not compatible with upgraded repositories, no chance).
- You have to take due care to use only supported DB and OS versions. Everything else will sooner or later cause severe trouble, be it while testing the upgrading or (even worse) after the final upgrade.
What you can do is to create a copy of each 9.1 repository by backing up and restoring its contents into an empty DBMS schema under 9.1; then you can upgrade the newly created copies under 9.6.1. This will allow you to run all your reconciliation tests before performing the "general" upgrade.
Keep in mind for such a parallel installation that usually Informatica will give you license keys to run both versions in parallel for at most 2 months (at least that's my latest information from two years ago). You will have to talk to your sales representative about this topic.
Q2, installing 9.6.1 directly instead of upgrading: in my experience blatant nonsense and extremely dangerous. You can do so in a playground environment, but it's a VERY bad idea for customer environments. I wouldn't do that under any circumstances.
It's a complex matter, but things can be simplified drastically if you're only upgrading PowerCenter and nothing else. Please confirm or correct whether the upgrade is only about PowerCenter or whether other Informatica products are involved as well.
Nevertheless, as I have the impression that you are not very experienced with this stuff, may I suggest that you hire some experienced consuitant to first analyse all your environments and second prepare the upgrade in a graceful and useful manner. The more thoroughly you plan and prepare your upgrade, the less pain will it cause you. There are so many details to plan, to test, to verify, and to organise that I cannot encourage you to do this without professional help.