is it safe to add ubuntu linux on my macbook pro?

Indeed you will require more of RAM if you have too many vm that require large amount of RAM.

You can definitely set the minimum RAM for each of the guest OS and give it a try.

Some linux distributions run with minimum 512MB RAM, check ubuntu requirements.

By the way what is the Macbook pro model/year you have?

Linux should work well on that laptop. However, I don't recommend sharing the main macOS boot drive with another OS as people almost always select the wrong sizes of the partitions for each OS which usually results in problems later on when a user tries to resize them. It is a bit safer to install Linux to an external drive. Even safer would be to run Linux in a Virtual Machine.

Not all Linux distributions may be Apple friendly when installed on bare metal. The easiest ones to try out would be one of the Ubuntu flavors such as Kubuntu or Ubuntu MATE (the default Ubuntu with the Gnome Desktop Environment usually needs more memory than the other flavors). Linux Mint is another good distribution for a beginner and it should just work on a Mac. Other distributions will likely work, but they may require more customization to work properly on a Mac on bare metal. If you are not familiar with Linux, then these are also good choices when used within a VM.

As per the spec you can boost your RAM by upgrading to 8GB.

If you really don't want to mess up with boot.

Suggest you to use VM<Virtual Machine> like Parallels Desktop, VMWare Fusion, VirtualBox, etc.