What is the difference between RMRK and KSM and how does it impact my use of Singular?
The difference between RMRK and KSM is not relevant to your use of Singular.
RMRK is a protocol that lives on top of the Kusama blockchain. Think of Kusama as a computer and RMRK as the software running on that computer. KSM is the token of the Kusama blockchain just as $RMRK is the token of the RMRK protocol.
The only thing to note is that (for now) you can only pay transaction fees on Singular in KSM. This is because each time you interact with Singular, you are interacting with the Kusama blockchain and so you need to pay for that interaction.