It's an alternative to the Feed-in Tariff scheme that is now only for larger systems and non-FiT technologies. Small renewables that were installed before the FiT started in April 2010 could sign up for ROCs but were all automatically transferred to FiTs when it started. Pays around 4.5p per kWh but some technologies have a multiplier. Still applies in Northern Ireland I think.
No, at present you have a one-off choice of whether to sign up for FiTs or ROCs for systems between 50kW and 5MW. Although DECC are talking today of changing the rules on this to make all sub 5MW systems have to use FiTs.
If you opt for FiTs for anything over 50kW then it is not subject to MCS control and is registered direct with OFGEM under the ROO-FIT process.