When adding a new rule to a security group, is it needed to restart the instance to the new rule take effect?
If you forgot to assign a security group rule to the security group, but you created it (the security group) and assigned it to the instance. There is no problem to update the security group with the new rule. After it, you could use it without no need to restart the instance.
For example, you can find in port 22, without it, you cannot access your instance via ssh. You need to add the rule and then you can access the instance via ssh.