Lineset sizing for multi-zone mini-split
I would do what the oem tells you but 3/8 on a 18k sounds a bit small to me but  the oem should know.
I am sure the last 18k I installed required 1/2” on the low and 1/4 on the high.
I would rather use the larger ports for your larger (18k) inside units as throttled suction line will reduce their efficiency.
yes you only have 42 max But if the return line is throttled on the larger ones they may suffer more because of the under sized suction line.
I may be out of line here as I don’t know the brand or control system but if in parallel with the flow controlled in the head unit go with a larger suction line on the 18k units and skip using the adapters
They would not make the suction line on the inside units 1/2” if it did not need it in my opinion. But I only do hvac part time.