Don't fall in love with me yet, we only recently met
The Card Sail says Place 1 stone on one ship and sail that ship ... Does the Stone count have to meet the required minimum amount of stones to sail?
Yes. This is specified in the rules.
And is the Sailing part optional if it does meet the minimum stone count for sailing
I would suggest saving your Sail card and just using the regular 'place 1 stone' option in that circumstance