I'm using RM as a downloads manager for files users want to download. I changed the labels accordingly.
Someone has produced an addon that uses RM to make a store/cart system. I assume I cannot use this, as I am already using RM for downloads. Correct? Or can I use it for something else also at the same time?
Out of the box, the RM can only be installed once per installation.
There may be an add-on which allows for multiple installations, which coupled with the other add-on may provide the functionality you want.
You will need to check with both add-on authors to confirm if they will work together.