If you use only ATUM free plugin (with no add-ons), you can use any POS plugin you want as ATUM itself shouldn't have any conflict there.
Only if you use Multi-Inventory or Product Levels, there could be conflicts. In this case, we cannot recommend any 3rd party POS plugin as we don't know any that is fully compatible with them.
Regarding the ATUM App POS module, this should work but we don't recommend using the beta version in a production environment. You can test it if you want though.