If you are worried about a switch failing wire them in a series. That way they both have to activate.
As for the RO top off: Its been done before. You need a solenoid valve hooked up to your RO/DI unit. When the float swith kicks on the valve opens, filling up your tank.
you could try a core & coil ballast. you can find them at your local greybar, hellolights.com, and other places. Do a search for DIY metal halides. You will find something on the subject.