custom ID to fieldset + 2 column grid for forms

Hi there,
i’d like to know if anyone can give some tipps for best practise, because i couldn’t found any reliable solution out of the box.

1. How to parse a custom css id to the fieldset wrapper “fieldsetstart”?

2. How to build a 2 (or more) column css grid for the form if needed (no global setting).

I’d like to set some css params to the fieldset like <fieldset ID=”XYZ” class=”abc”> …

Can anyone give me some tipps or maybe a hook for that issue. If so i’d like to complete my issue and share my code right her. Thanks a lot :)!!!!