The keeping of black/silver arowana with large aggressive SA/CA cichlids is a a pretty common combination. Where you can get into trouble is with having the arowana too small. If the arowana is under 6" I would not recommend having it with any tankmates. Once you grow the arowana out to 10-11" you can mix with cichlids, but keep the length of the cichlids to about half of the arowana's length. Between 6 and 10" is a gray area, you may be able to do the mix, but the safer path would be to wait a couple months for the aro to be 10".