I was quite anxious when teaching my first solo fundamentals about whether I would know who were suitable trainees to recruit, and who, if any, were just too distressed or unaware to cope in a group training. I'm grateful to several established teachers who helped me to think this through, and particularly to those who emphasised that I could probably put some trust in applicants themselves to know their own limits and capabilities. I was thinking about this particularly in the context of having advertised the fundamentals through (among other places) a charity supporting mental health service users.
Because of Dorset being a rural county, and the recruiting for the course being rather close to the start date, I decided against trying to meet all the applicants before the course, and took two steps to reassure myself:  I sent them a booking form, modelled partly on one Richard Mills has published, on which I asked them specifically about their needs. See the attached file.  I treated the first day of the course as a 'come and try it' day, with the option to leave and have a refund at the end of the day if coco didn't seem to be right for the person. In the end I didn't need to use this option.
After I'd adopted this method, I noticed some pretty trenchant remarks in John Heron's Co-Counselling Teachers' Manual about the need to screen, e.g. "There needs to be a very large number of the best functioning people in co-counselling before there will be sufficient human and other resources to handle the worst functioning people. To try to help prematurely the worst victims of the old system will forever subvert the establishment of a viable alternative system." Plus some very good advice about how to go about it if you do decide to meet all the applicants before the course.
Perhaps I was lucky - all my trainees seemed to get along just fine in the group. Interestingly, of the two who disclosed nothing personal about themselves on the booking form, one left the course early and the other was the only one who I felt struggled a little. As regards the others, I felt that the information on the booking form gave me the chance to have a more useful exploratory phone call with them before the course than I would have had without it. So - so far! - I'm inclined to recommend the booking form as a useful tool.