Hubitat has various built in generic drivers that should help get most devices paired. Your somfy z-wave will probably pair up as a dimmer or you can switch it to that. Not sure about the pool controls.
We do offer a driver editor that is based in groovy and allows for user drivers to be created. Once you get your hub, our private area for hub owners will be available and you can learn and ask questions there about how that process works.
I’m trying to create a custom device type (really trying to modify an existing SmartThings DTH), but the total lack of documentation makes this hard. Are there examples of custom DTHs? For example, in ST, you can do a POST with physicalgraph.device.HubAction, but that class doesn’t exist in Hubitat. I love the idea of the hub, but the ability to extend it doesn’t mean much if nobody knows how.