Hello again.
With your current setup I would recommend you set the Epson large format and the older HP connecting via wifi to DHCP. This action will let the Time Capsule allocate an IP address that is not being used by other devices that are also connecting to the Time Capsule - either via Ethernet or wifi. And it will also let all devices print to any printer regardless of the Mac Pro being on or booted into Windows.
As for the new HP that is connected via USB to the Time Capsule, you do not need to make any network setting to this printer. The Time Capsule will share the USB connection to the HP via Bonjour, meaning that other computers including iOS devices will see the advertised printer.
Once you have configured the Epson and old HP to DHCP, please restart them so that they can obtain an IP address from the Time Capsule. With this completed, you can remove their printer icon from the Printers preference pane and add them again, as they may now have a different IP address and the current printer will not work if that is the case. Note that the new HP attached via USB to the Time Capsule will not have changed, so there should be no need to remove it from the Printers preference pane and add again. It should just work as before.
Re the question regarding static IP, there should be no need to do this providing all devices on your network can communicate with the Time Capsule router to obtain an IP address for your network. It is only when a device has an issue with using DHCP that you may need to play with static IP's.
As for printing from the iPhone when you are away from your wireless network, the email approach is easier and safer if you can get it working. Apple has the Back to My Mac service with the Airport routers but I don't think you can use this service to print - even to the USB connected printer - and reading the specs I think you cannot use it with the iPhone. And for the Mac I think you need to have OS X 10.7.4 minimum. The other option would be to have some sort of VPN service that you can securely connect to when you're not home. I use this from home to my company's servers using Cisco AnyConnect but have never set such a thing up to access my home Mac or printer from my iPhone when I'm out. So if you cannot get the HP email thing to work then try creating a new post about this topic.