Are you saying that adding text to the value of a booking field Constant, from the language override, is the best way to add an explanation/tooltip?
Let's talk me through this, let's say your caption is COM_MATUKIO_CITY, then with _DESC it becomes COM_MATUKIO_CITY_DESC - this one can be used for long descriptions (tooltips on mouse over) with an language override:
docs.joomla.org/J2.5:Language_Overrides_in_Joomla
e.g. just set COM_MATUKIO_CITY_DESC = My nice tooltip text
I know this is far from optimal, this is just a workaround suggestion as long as there are no better tooltips in the new form (coming with Matukio 5)
Is it your intention to eventually migrate all the features from the old template to the new template and eliminate the old template altogether?
No, remove is the wrong word

Going to keep the style but merge the code in all three (one more to come) templates, so that they all share the same functionality just have a different layout (which will also be much easier editable, because most application logic will be removed out of the default template).
Kind regards,
Yves
P.S.:
250 characters would not be necessary in many cases (though varchar is for variable-length strings of up to 65,535 characters used with 5.0.3 and greater MySQL databases.
Yeah, i know but it's a field varchar with a length val of 255 currently

Would need to alter the table field varchar (255) to varchar (65534) - no big deal, when necessary