Carrier Aggregation PCC rule and spacing for contiguous CC

Hi all.
When we have a particular Carrier Aggregation combination like 3A-7C, is it possible have the PCC in any Carrier?

And about contiguous CCs, is it necessary have overlapping edges (one CC upper edge = other CC lower edge) or is just sufficient have the spacing between center frequencies = N x 300KHz?
The rule for contiguous CCs (spacing between the centre frequencies = Nx300 kHz, N=integer) is kind of confusing because it is possible have gaps between CCs.

Thanks