I feel your pain and confusion Paul, I'm just at the point where I needed a new pump and was looking for a replacement for an XF250.
1. The 300 series pump has different fittings as you have discovered, a new controller is required for the pump and whilst the controller 'controls' the pump, its main purpose is as the power supply.
With a 200 series pump and controller, and a 300 series pump and controller, you should be able to control both pumps using an ICV6. The ICV6 is designed to act as a network hub for Maxspect products.
I reckon a viable option for you is to follow Angie Brumfields comments earlier and opt to buy a Wave Engine https://www.coralvue.com/hydros-waveengine-3537
Options.
1. Return the XF350 pump and source an XF250 pump - the simplest option. You can then just plug in the new XF250 pump to your existing controller.
2. Keep the XF350 pump but you will also need to buy a XF350 Controller. With this option, when your existing XF250 pump retires you can buy new XF350 pump to use alongside your original one on the XF350 controller, and then sell on the 200 series Advanced Controller.
3. Sell your ICV6 and 200 series Advanced Controller (if you can), and buy a Hydros WaveEngine. This will power both XF250 and XF350 pumps (doing away with the Advanced controller and multiple power bricks). There is a Hydros app you can then use to control both pumps, and optionally EcoTech Vortech and Vectra pumps.
Make sense?
I don't work for CoralVue but am interested in how you get on, happy reefing.