Ahh this is a huge rabbit hole - welcome to the fun
You might want to read up on some stack and reach - tons of info on stack and reach over on Slowtwitch in the fitting areas or just google it.
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Yes, the front end geometry is key, but ultimately getting fit first by someone who does Tri fits will get you the stack and reach numbers to make sure you purchase the right bike size. Some fitters can even narrow down the best bike choices for you eliminating some that will be a struggle to fit correctly (IE not having to swap stems, aero bar risers, etc.)
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No, depends if you are going to ride TT sometimes or not. If you are only doing Tri - the tri bikes will likely have aero options (frames) not available under UCI and/or drink/storage options built in or attachable to the bike. IMO generally fit wise they are pretty much similar, for example some bikes will have a different seatpost for TT versus Tri fitting. IMO it was be pretty easy to achieve the same fore/aft on a P5, shiv or plasma unless you are oddly shaped (which you said you were not), as the bikes fore/aft are not that dissimilar, its the stack that seems to be the most different and varies from brand and their fitting philosophy.