If you're interviewing for someone you will be working with, picking someone you feel comfortable with is NOT inappropriate - it should ensure a smooth working relationship.
I can't stress how important this is. If you are not comfortable talking with them or spending time with them, your working situation will not be real pleasant for either of you and may create stress.
Also, if you will not be the only person working closely with this individual, you should have the other people who will be working closely with them sit in for a few minutes or spend some time talking to them before/after the actual interview.
Have someone greeting the applicants - they should chat to them, make them a coffee, relax them and show that it's a good place to work. After the interviews, compare notes with the greeter to see if you agree. Some people are different in the formal interview.
This would be an excellent role for someone else who would be working with the new hire. If the current employee is not comfortable with them, it could cause problems.
Although it did not happen by design, when I interviewed for my current job this wound up happening. My interview was scheduled for 830 on a Monday. While I was early, the interviewer got caught in traffic do to an accident and wound up being 20 minutes late. So, she called a gentleman who was already doing the job that I was interviewing for and had him come out and talk to me over some coffee. I had a job offer that afternoon from them based partially on what he told her.
When he turned in his resignation a little over a month later, the boss remembered what had happened and had both of us sit in on the interviews that she conducted for his replacement and hired the guy we were most comfortable with.
Another important thing to do is to get them talking about their life outside of work. A person who leads a balanced life will be happier. Happier people tend to perform better. Likewise, a person who is not pursuing other interests is likely to burn out and leave you looking to replace them in a year or two. Ask them what they do to relax or what their hobbies are.