I did live in Kelso for a few months in the early 1980’s the feed in tariffs are not so generous these days hence the installer need to price accordingly.
Top my head I think my panels are 370 kW, I have seen them momently spike over 5 kW on one occasion the solar controller however limits the output to 4.44 kW. Apparently the further south you are the better the output.
If you add some additional panels you will need an additional invertor.
Unfortunately you cannot mis match solar panels, solar panels are funny thing if you connect a 100w and 80w panel together you will only achieve 160w max not 180w ?
Unfortunately I have suffered a brain injury and occasionally I get confused and often say the wrong thing.