ZaOracle

Previous post from @AndrewJdc is spot on.
You can have 6 on each MPPT and even if the sizes vary a bit it will be OK as we don't always have the full 1000W/m² of irradiation. 
With 6 you are about 200W per MPPT over the spec of the inverter. I think this won't be a problem. 

Golden rule orientation and tilt must be the same on a single input mppt( 1 string/mppt) for best performance. On a 2 string mppt you can have 1 string of equal amount of panels facing east and the same amount facing west into 1 mppt with 2 strings input as they are paralleled entering the mppt this way you don't have peak production at mid-day but you have an equal spread throughout the day.

You have 5 x 550w/ mppt = 2750w/ mppt currently. Now you are asking 7 x 550w= 3850w for mppt1 and 5 x 550w for mppt2. You will be 600w over the spec for mppt1 but that will not be a problem for the Sunsynk 5kw. 2nd question no the 2 strings on different mppts do not have to be equal.