I recently visited a K-8 Montessori School and love their methods. They combine first through third grade into one classroom, fourth through sixth grade into another class, and seventh and eighth grade into the oldest class. Kids also learn at their own pace. My daughter's reading is advanced, and she's very artistic, but she is very tiny so we do not plan to skip her any grades. Are Montessori schools good for gifted students? How can we still keep her with her peers after eighth grade?
Montessori programs can be wonderful for gifted children, especially given your daughter's needs. Multi-age grouping will allow her to be stimulated but will keep her with age-appropriate peers. She's likely to move more quickly through the materials but in this setting she'll be able to have the flexibility to explore other areas she becomes interested in and to work with her art.
The major problem will come -- as you also note -- when she finishes eighth grade. At this point, however, we can't know what to do then. It will depend on what her needs are, and what the options are. A lot can change between now and then, so it's better to look at her needs today and see how they can best be addressed. You'll be able to watch her grow and can plan further along as her needs change. For now, it seems like you've found a wonderful place for her to start!