Hi Anthony,
A very interesting question!
Yes, you can add extra panels to your roof, but there are a few complications in doing this.
To take an example: if you had installed an eight panel PV system (1.84 kWp) on your roof now and then wanted to add an extra four panels (to get you to 2.76 kWp) in September 2012, you would have to go through the Feed in Tariff application process twice to ensure both installations are compliant with Ofgem's regulations.
This also means that you will potentially get two different Feed in Tariff rates: 43.3p for the eight panel system you installed now and a lower rate for the four panel extension you installed in 2012.
Considering the expense of sending an installer out twice to your home, it would probably be more cost effective to finance the larger installation in one go either through a personal loan or a loan through your installer.
To answer your second question, your tariff and your Feed in Tariff are entirely independent of each other. You can have a green tariff like Juice or National Trust Green Energy from npower and still claim Feed in Tariffs for your microgeneration system without any penalty.
