RAC is a option of Oracle Software. You can't Install Oracle RAC Single, because it's not exists.
Since you have Oracle Clusterware Installed/Configured, what you can do :
* Install Oracle Database Software with RAC Option Enabled.
* Install Oracle Database Software without RAC Option Enabled (i.e Non-RAC/ Single). One Installation per node, same as Single Node
* Install both RAC/Non-RAC on same Clusterware. (One or multiples installation for each type)
* Create a Single Instance on Oracle Database with RAC or Oracle Database without RAC
* Create a RAC instance only on Oracle Database with RAC
All Options above is supported by Oracle.
You can certainly create a single instance database on a Oracle cluster. They can coexist quite nicely with RAC databases on that same cluster. This is not that much different than having multiple single instances running on the same server. You will need the resource to support all instances no matter if they are RAC or single-instance.
But you will want to take heed of what happens if that node goes down. You won't be able to access the single-instance database until that node returns.