it can be really tricky.
we have never wanted mainstream for dd1, really. we did entertain the though for a bit, and she went ot mainstream pre-school, but it was always with a view to collecting evidence as to why she needed to be in ss. LA wanted ehr in mainstream, as she was 'no trouble' 
dd1 has also been to a ss placement which was so wrong for her (again, to build up evidence to support what we knew was right for her). the first school that was right for dd1 was tiny - dd1 was pupil number 3
- as what she needed at that point was no children (she was scared of children). she had her own classroom, and it was exactly what she neded at that point.
after nearly 2 years there, she moved on, and is now at a small ss, in a class of about 8 (20ish in the school overall), and it is right for her to be there now. when we looked around when she was 4, it was not right for her, as she would have ben terrified of the other children. now she can cope with that (thanks to her last school), it is right for her.
but as I said, we saw many, many schools which had absolutely nothing worng with them - some of them I really liked, but they were not right for dd1. it would have made all of our lives easier (except for dd1's) if we could have sent her to some of those schools - we owuldn't have had to move house (again!), we woudln't have had to take the LA to tribunal, etc. but it would not hav been the right decision for dd1.
you now know a little more what to expect, and what you like/don't like about ss. keep looking. they vary a lot in terms of management, ime.