Is there any reason you're not side venting it?
I'd be surprised if Paloma didn't have a termination cap to deal with concentric vertically vented installations, but if you ran the intake & exhausts separately you can use an inverted U the top to prevent rain penetration. See page 8. A U at the top of the termination kit you bought may have issues with exhaust mixing with the intake- don't do it unless you find it in the fine-print, but a simpler rain-cap might be OK.
If you're replacing a 50 gallon tank you'll almost certainly have to upgrade the gas plumbing to a bigger diameter for the 199KBTU/hr burner. Don't be fooled by the 3/4" gas hookup.
With 3/4" gas piping the tankless has to be within 20' of plumbing from the gas regulator, and that's counting ever ell as 2.06' and every tee as 4.12'.
With 1" you can push that to ~60', counting ells at 2.6' and tees at 5.12'.
Most big-burner tankless heater installations will need 1-1/4" to work (as in work well), 1-1/2" or more if there are any other loads teed along the path (either before or after the tankless.)
You may be better off calling in a pro to install it- somebody who has the tools knowledge and experience for debugging it. It's more expensive to pay somebody to rip it out and start over than to get a good plan of attack ahead of time. You may be able to find somebody willing to give on-site guidance, tell you what you need and let you do the work, but they should also inspect it before you turn the gas valve and throw the power switch This burner is about 4-6x the burner of a tank heater, and bigger than that on most home-heating boilers & furnaces, and being a modulating burner it's more sensitive to the supply line pressure (and pressure spikes/drops) than dumb 80% bang-bang on/off furnace burners. The fuel pressure drops are best measured under a variety of conditions and combustion efficiency tested as part of any real commissioning of the unit. There are a thousand ways that it can kinda-work or be almost-right and still bite you in the end. (In some states DIYers aren't even allowed to do their own gas line plumbing- not sure what the status is in FL.)