Oracle & Grid software owner should be same or different
What do you already have currently? I see no reason to change the current ownership configuration just because you are going to add single instances. Unless there is a specific need for separate users, eg. auditing purposes, or a different DBA team is to manage the single instances.
Oracle Home should be same or different (Oracle version is same here)
If the version is the same, the Oracle Database Home can be the same.
What are the best practices in such installation cases
Generally there isn't much difference from installing on a standalone server. One consideration would maybe creating a separate listener for the single instance DBs
Do we require to install another ASM & GRID home or it can be managed in the same installed & running cluster system?
You can use the current Grid home to manage the ASM instance for the single instance databases.
How ASM would be connecting, suppose we install db on db node1 then ASM sid would be +ASM1 or just +ASM?
Other things to take care in such case
Are the new databases for production use as well? Performance, capacity, etc. Be aware that if all your databases are running from the same Oracle Home. What if you need to upgrade the DB version, and one of your apps running on the single instance isn't certified on that version? Then you might want to consider using different Oracle Homes.
Use the same GRID and DB user. Just use DBCA to create a Stand Alone Database, that´s all.
If you use DBCA, you can choose all the configurations and you choose there ASM to create your database.
The reason to use another OS user, or another Oracle_Home, for example: if you have an EBS environment on one node of your RAC (Stand Alone Database), then yes use another S.O. User, another Listener and another Oracle_Home, so you can patch, restart services without affecting the RACs Databases.