Q1. Can this handle a 240v 5,500 watt water heater? 25-30 amps on each of the 2 legs?
A: If you look at one of the photos of the inside cover, it shows 40A @277 Resistive load.
So "YES" it will for a water heater

Q2. If you want to separately control a pool pump and a pool heater, can two of these be installed and then be controlled separately from the app?
A: Yes, they can be connected together, and be controlled separately by the app.

Q3. If i want to control 4 devices with this, how many units do i need? pool pump (220v), pump for sheers (220v), pool cleaner (220v) and pool light (120v)
A: To aviod overload, you had better use 4 smart box, because of large voltage fluctuations when pumps ON, and meanwhile please do not use both 120V and 220V in one device.

Q4. Shows both nc & no set of contacts - is that correct? looking to wire my dual speed pool pump so that it's always running, nc = low, no =high
A: The item can only turn the device on/off. It can not adjust the speed of your device. If you need a wiring diagram for your device, please tell us the specification of your device.