After spending nearly 7 months on English.SE, Interpersonal.SE and a few other sites here, I think the Stack Exchange network has evolved over the years to accept varying levels of subjectivity depending on the subject matter of different websites.
For example, English.SE is more subjective than Stack Overflow but more 'objective' than Interpersonal.SE; History.SE is relatively more 'objective' than Politics.SE but both are much more subjective than any technology-based SE; and the level of subjectivity certainly seems dependent on the subject matter of the website.
Regardless of which, the Interpersonal Skills community (IPS.SE) sets just as stringent standards as any other SE site in determining which questions are 'off topic', 'too broad', 'bad subjective', or 'primarily opinion based compared to acceptable limits for this subject' and which answers are off-topic, low quality or just speculative without being properly backed up by either experience or references.
Are there any non-opinion based questions/answers regarding interpersonal skills? (...) what is the ideal for non-opinion based questions and answers here?
Look at it as a site on SE that provides authoritative Q & A based knowledge-sharing on life skills as applied to interpersonal issues -- questions would be necessarily subjective to some extent here, but do need to be as objective as possible in tone and content, and also need to specify what the OP wants to achieve in the context of the particular problem.
Answers might also be by necessity based on personal or cultural opinion to some extent, but again, answers cannot make wildly opinion-based suggestions but need to back it up with credible personal experience (preferred in the context of IPS) or references; and also need to be on-topic for this site.
Questions that invite primarily opinion based answers (even considering the more subjective scope of IPS) will be closed, and answers that provide mere opinion not properly supported by experience or references will be deleted. Of course both types of posts can still be edited and will be undeleted/reopened when they attain standards of objectivity acceptable to this community.
(...) Is that just decided by votes?
Community consensus on content and quality is certainly expressed by upvoting, downvoting, close-voting and delete-voting, just as anywhere else on Stack Exchange. In addition, the site moderators and Stack Exchange Community Managers are very vigilant to ensure that posts on IPS follow the broad structure of the SE model and do not devolve into mainly opinion-based debates or discussions.
Questions, answers and comments cannot be used for general discussion and answers cannot be posted in the comments section (at risk of deletion). Comments that simply express opinions without seeking to clarify or improve the post will also be summarily deleted. Contentious posts attracting large scale discussion/debate can also be locked to stop acrimonious or divisive debates on sensitive issues.
All these measures are intended to keep this site as objective and problem-oriented as possible, closely following the generic SE model.
Furthermore, as mentioned in @apaul's answer on the original question, the pre-existence of Parenting.SE and Workplace.SE sets a solid precedence for the necessarily-to-some-extent-opinion-based character of Interpersonal.SE!