do your research on the type of nem you want. Some require bigger tanks than others; carpet anemones can grow to over 18 in, and require a minimum of 100 gal. Others stay much smaller, and will do fine in a 20 or 30 gal. Other than size, yes, at 9 months you're probably good to go. Whether or not the clowns will host, you won't know until you put a nem in. Some do, some don't, and different types of clowns supposedly prefer different types of nems.