I am adding instances to default subnet . Is it possible to do ssh between instances in default subnet using private IP
Sounds like you are going to do the "jump box" thing. You might want to look into the Session Manager feature of AWS Systems Manager. Session Manager lets you establish a terminal session directly to any EC2 instance that has properly registered with Systems Manager, without any need for network configuration or keypairs (the local agent might need to be updated, which is really easy via Systems Manager).
I need to make network configuration . Any changes required to set up in Security groups .I also generated ssh keys using sshkeygen and added in authorized keys. Still ping fails
if you want to ping, you need to make sure that the appropriate security groups allow ICMP. Arguably, you can just make a single ICMP security group, temporarily add that SG to your instances to check connectivity, and then remove that group.