
Karen, #5 and #8 perle on 22ct hardanger fabric is more or less the "standard" combination and on the whole I've had no problems with it, except perhaps when there was a lot of contrast between fabric and thread (e.g. white on black, or dark blue on cream). Could it be that you are pulling the stitches a bit too tight? If you try to keep the tension a bit looser the threads may sit more plumply on the fabric.
Coverage is definitely good on 25ct, and on both fabrics 1 strand of Caron Watercolours (instead of #5) and Wildflowers (or Impressions) instead of #8 look lovely too.
I'm still working on that experiment with lots of different fabric/thread combinations, you can see the first few (with comments) in my Hardanger thread.