01 See #TiSDD 3.2, Personas, and #TiSDD chapter 10, Facilitating workshops, for hands-on tips on facilitation and how to build a safe space.
The quality of the results of any co-creative workshop depends on the knowledge of the workshop participants. In this case, it will depend on how much participants know about the group of people you want to exemplify with personas. For example, if you want to create personas of customers you would do well to invite frontline employees who are in direct contact with customers every day. Be careful if you conduct a co-creative workshop with people who do not have sound knowledge or only a superficial or abstract knowledge of the subject matter. The results might look convincing, but often they are very biased. For example, if a marketing team without prior qualitative research and without deep knowledge of the daily lives of customers conduct a co-creative workshop on personas, the outcomes tend to represent their ideal customers. Using such idealized personas as a basis for a design process is risky, as you could end up with concepts that lack a customer base. 
In addition to the know-how of the workshop participants, a second important factor for any co-creative workshop is the qualitative research you do prior to such workshops. As a rule of thumb, the more valuable data you bring to a co-creative workshop, the more representative your outcome will be.