Hi there;

I just wondering if I can restore a RB951G from a RB951 backup, because I clone the configuration settings from my old RB951.

Regards.

Generally binary backups should only be restored on very same unit that produced backups. The reason is that binary backup contains also MAC addresses and running two units with same MAC addresses in same network will cause trouble even if those devices are of exactly the same hardware model.
The problems when restoring backups on different device models are even bigger ... starting from different number of interfaces, different wireless chips, ...

So when one wants to create same configuration on different hardware, it's best to create text export of configuration (execute command /export file=oldconfig from terminal window and fetch the file). Beware that some things are missing from text export, such as user, passwords, certificates, ...
It is then advisable to upgrade the new device to latest long-term (or stable) ROS version, reset to factory default settings and adjust the settings to match the functionality of the old unit. Using the old text config export as a reminder / cheat sheet ... modern ROS comes with pretty good default settings (specially true for firewall rules) and migrating configuration to new device is a great opportunity to clear up the setup a bit.