I dont think mixing the sand and mts would hurt anything. Its possible that the dirt will settle back down below the sand over time, but im not 100% on that. I think your best bet is to use whatever sand you find pleasing to the eye and just make sure you have a good amount of flow/filtration to pick up debris/mulm. Sand doesnt really hold mulm all that well especially as it gets settled in, so I would worry to much about grain size, more so about filtration. Keeping a cleaning crew of maybe otos or some shrimp would really help too because they will constantly clean the plants, keeping the mulm stirred into the water column to be caught by your filter.
So my advice with you is if your plants are growing well and you like the looks of the substrate you already have, get a small cleaning crew and upgrade your filtration before you swap it for gravel, and maybe a heavier sand would be best. If you dont want to fool with removing the moon sand I would go ahead and mix it up as long as you dont mind that you may have a triple strata substrate in the future.
What filter do you have on the tank?