I agree absolutely what is already said: work for even stitches. That is the most important thing. Your stitch length will vary with the different fabrics and the different battings you use and probably it will change with increasing practice and skill...but just do it your own way and have fun.
I did many hand quilting demonstrations on exhibits and fairs over the years and many people told me: your quilting stitches are great, they look like machine stitching! At the beginning I felt a little angry about these statements because people seemed to be unable to distinguish between hand quilting and machine quilting! Today I understand it as a compliment, but I don't aim for it.
Regarding the preprinted top: Several years ago I quilted a preprinted top and after the quilting was finished I soaked the quilt in clear, cold water for a while. This was suggested by the producer of the top and the ink disappeared completely. Heat can be a problem as far as I know. Maybe Holice can help you with that?