What is the wire spacing for the floor of the cage? If it is greater than 1/4" x 1/4" you won't be able to use it.
I'm not sure what you mean by removable grate but I suspect it's a wire shelf or ramp. In any case just don't use it. There is no need for it and ramps and wire shelves are notorious for breaking chinchillas' legs because they can get a part of a foot through it as they come down over the side. Sometimes this is fatal. It happens a lot.
I strongly recommend installing at least one solid wood shelf in the cage. It it better for chinchillas to rest on these and they'll usually sleep on them. A properly sized cage will allow them to jump from shelf to shelf. They can jump about 6' straight up.
You'll want to use pine chips under the floor of the cage in a solid pan to catch the droppings. Typically chinchillas will choose one corner of the cage to pee in. It might not happen right away but you'll see a pattern after a couple of weeks once they are in a new cage. If you pile all the wet chips in one corner that should help.
You can try to put chips in a pan. I know some people do that to help train a chinchilla that won't do that on its own. Just keep the pan in the same place for a few weeks and when you remove it the animal should still use that corner.
Hope this helps.
