It would be better to set the radiator system and ufh system on seperate thermostats. Because they will heat up and cool at different rates.

There is nothing wrong with using nests although rather costly for each of the zones but you should treat the radiator portion as a zone meaning you will need 5 thermostats,
You can put thermostatic radiator valves on but you will still need some means of timing the so either a 5th nest or an separate programmer just for the radiators.

When you fit your ufh each zone will have a thermostat you need the electronic type which will work with most stats including the nest and hive very expensive to have them in every room but they will work

Yes, that shouldn't be a problem but I would advise factoring it in at the design stage rather than afterwards to avoid any potential incompatibility issues.
Also, as suggested in another answer, the 1st floor with radiators should be controlled by a Nest too. In all you will have 5 zones.