It is rarely necessary to clean Gerbils out more than once a week or fortnight. Gerbils, like most rodents have a strong chewing instinct. They can keep their teeth at the proper length by grinding them together unless the teeth do not line up properly, but still instinctively chew on anything they can get their teeth on. Cardboard or shop bought chew sticks are OK, but the cheapest and best thing to do is to use branches and bark from trees in your area. Fruit tree prunings are ideal.
Pine cones are a great hit too.
This isn't really a good idea. Whilst gerbils are not afraid of large predators like cats and dogs, these animals can easily injure or kill a gerbil. Even contact between different pets of a similar size can be a major mistake. Gerbils tend to be extremely aggressive towards other small rodents. They will normally attack and attempt to kill hamsters, rats and mice.