You can add your own sprites and/or edit the ones you bought to use in your game however you please :)
The thing that's not allowed is exporting assets from MonMae to use them in a elsewhere. If you're making a game with MonMae, you can add or edit sprites as you please.
If you buy artwork in one of my bundles or asset packs, you can edit them or use them together with other assets for your game as you please~
If you have any more questions feel free to contact me in our discord channel (it's linked on this page). I can respond much quicker on there :)