Sources- Talking Hands by M. Fox & an ASL class I took.
I see you are sold on the idea of raising a bilingual kid-- me too, baby just arrived last month, and we're doing Russian, English with ASL. Since I live up the road from Gallaudet, I thought might as well learn real ASL and not baby ASL. Baby ASL is anywhere from 20-50 signs that are used just when baby is in that time gap between being able to learn & use signs but can't enunciate a spoken language.
Your question really pivots on if ASL is a language. Amazingly, before around 1950, academics didn't consider sign languages to be real languages-- hard to say what they thought, but probably they imagined it fell in the range of gesture systems like baseball signs, or what a street mime does, or something like that. Another hurdle ASL had to over come was if it was just a way of "writing" English with your hands.
Nowadays there is no question that ASL is a language, and distinct from English. The grammar is quite alien as compared to English or any other European language you may have studied in high school. It's topic-comment, not subject-object-verb, adjective strings follow then noun (but single adjectives can go before or after the noun). In a lot of jurisdictions it counts as a language for foreign language.
I think the boundary between parents using baby ASL and those raising bilingual sign/English children is:
- sheer quantity of vocabulary, baby ASL tops out at 50 or so words, conversational ASL requires 500-1000+ words just to squeak by
- usage of ASL word order & grammatical markers (esp the grammatical facial expressions).
- if you can code switch (i.e. switch to ASL without talking and still be understood)
Anyhow, as experiments go, the research says it is hard to do it badly enough to cause harm and the main risk is that the child just doesn't learn the minority language.