A few things that you need to consider when mixing species. Whether you can provide optimum living conditions for all species, whether they may cause harm to the other species and what your reasoning is for keeping those species together.
Rainbow sharks are tropical fish and not possible to keep both the axolotls and rainbow sharks in their optimum living conditions. Also, the fish may be eaten by the axolotl or cause harm to the axolotl by nibbling at gills.