Yes, a lot has been done, not only with miniature sinns but with larger sinningias. There are all kinds of hybrids out there. I remember one of the first whites was called 'Premier White' but I don't know what there is these days. Marcia Belisle, has a mail order list with quite a long list of mini sinningias. She does not have her list on line but can be emailed. Her link is on the suppliers page at the aggs.org web site.

Jon