Updated after reading OP again
I'm not sure you can change one without the other, my quick fix suggestion would be to add a guest condition to
editor_toolbar_off template and add the
pattern and
required attributes to the textarea:
Find:
Code:
tabindex="1">$newpost[message]
Before add:
Code:
<if condition="$show['guest']">pattern=".{10,}" required </if>
Then change 10 to whatever amount you wish min to be.
This is just a quick fix, I'm sure someone else would come up with a better solution.