Should I enter my best SAT section scores or best overall score from one sitting?

It's completely up to you.  We encourage you to try both as part of your planning and see if it makes a difference.  

Is the SAT writing score required?

No.  If you've taken the new SAT, simply provide your reading and math scores.  If you've taken the old SAT, we recommend that you include your writing score as it will help us do a better job of assessing your academic competitiveness at the schools on your list.

Is it okay if I enter both my SAT and ACT scores?

Yes, if you enter both, College Kickstart will consider both as part of its analysis (along with rank and GPA if you provide it).