I do my contacts with the negs in Printfile brand sleeves and don't have a problem with sufficient sharpness for the purpose. The key is to use a decent (and clean) piece of glass to hold them down. I have glass from a frame and just press the edges outside the paper as it exposes. I never had good results using the contact printers we had in school. The foam backing never really allowed good pressure, I'd guess.
If I were doing a contact print as a final print, I wouldn't use the sleeves.
Timing wise, I have the enlarger set so it's just outside the size I need, with an empty neg carrier in it and focused. I use a #2 contrast filter, f8 and 8 seconds. If I've underexposed the roll, sometimes I'll go to f11.
I develop my paper by time, not appearance. It's easier to be consistent that way. For RC, I use 1 minute and for fiber I use 2 minutes (I'm using Ilford paper and developer, so it could be different for different brands - read the info sheets on what you're using).