Oh, that's too bad. I have never adjusted the preferred release type sliders myself, so maybe someone else could give the specifics, but it seems to me that they should offer the options you need. If you're scanning singles specifically, have you set the singles slider to max and also set the compilation slider to min? It seems reasonable to me that compilations shouldn't show up at all if that's the case, but I'm not sure if they do or not.
Of course, you will need to make sure that the single releases exist in MusicBrainz if you expect them to show up. If you're using the scan functionality, the recordings attached to the single will also need to have fingerprints attached for them to show up.
I'm not sure how familiar with Picard you are, so please forgive me if I'm my description is too thorough (or too complex)