HELP! Mavericks and Mojave on MacBook Pro Mid 2012?

A mid 2012 MBP gives you 3 higher options for OS, but be aware Catalina would require formating a dive to APFS which Mavericks can't handle, Mojave should be OK...

macOS Sierra - Technical Specifications

General Requirements

• OS X 10.7.5 or later
• 2GB of memory
• 8.8GB of available storage
• Some features require an Apple ID; terms apply.
• Some features require a compatible Internet service provider; fees may apply.

Mac Hardware Requirements

For details about your Mac model, click the Apple icon at the top left of your screen, choose About This Mac, then choose More Info. These Mac models are compatible with macOS Sierra:

• MacBook (Late 2009 or newer)
• MacBook Pro (Mid 2010 or newer)
• MacBook Air (Late 2010 or newer)
• Mac mini (Mid 2010 or newer)
• iMac (Late 2009 or newer)
• Mac Pro (Mid 2010 or newer)

macOS Sierra - Technical Specifications

https://itunes.apple.com/us/app/macos-sierra/id1127487414?ls=1&mt=12

https://discussions.apple.com/thread/253906084?page=1

Fix for 10.12 createinstallmedia "not a v… - Apple Community

macOS High Sierra - Technical Specifications

General Requirements

• OS X 10.8 or later
• 2GB of memory
• 14.3GB of available storage to perform upgrade*
• Some features require an Apple ID; terms apply.
• Some features require a compatible Internet service provider; fees may apply.

Mac Hardware Requirements

For details about your Mac model, click the Apple icon at the top left of your screen, choose About This Mac. These Mac models are compatible with macOS High Sierra:

• MacBook (Late 2009 or newer)
• MacBook Pro (Mid 2010 or newer)
• MacBook Air (Late 2010 or newer)
• Mac mini (Mid 2010 or newer)
• iMac (Late 2009 or newer)
• Mac Pro (Mid 2010 or newer)

macOS High Sierra - Technical Specifications

https://itunes.apple.com/app/macos-high-sierra/id1246284741?mt=12

macOS Mojave 10.14.6 - Technical Specifications

General Requirements

OS X 10.8 or later

2GB of memory

12.5GB of available storage (OS X El Capitan 10.11.5 or later)*

Some features require an Apple ID; terms apply.

Some features require a compatible Internet service provider; fees may apply.

Mac Hardware Requirements

For details about your Mac model, click the Apple icon at the top left of your screen and choose About This Mac. These Mac models are compatible with macOS Mojave:

MacBook (Early 2015 or newer)

MacBook Air (Mid 2012 or newer)

MacBook Pro (Mid 2012 or newer)

Mac mini (Late 2012 or newer)

iMac (Late 2012 or newer)

iMac Pro (2017)

Mac Pro (Late 2013; Mid 2010 and Mid 2012 models with recommended Metal-capable graphics cards)

Don't use FireFox for these links…

https://itunes.apple.com/us/app/macos-mojave/id1398502828?mt=12

macOS Mojave - Technical Specifications

https://itunes.apple.com/us/app/macos-mojave/id1398502828?mt=12

Don't use FireFox for these links…

macOS Catalina - Technical Specifications

General Requirements

• OS X 10.9 or later
• 4GB of memory
• 12.5GB of available storage (OS X El Capitan 10.11.5 or later)*
• Some features require an Apple ID; terms apply.
• Some features require a compatible internet service provider; fees may apply.

Mac Hardware Requirements

For details about your Mac model, click the Apple icon at the top left of your screen and choose About This Mac. These Mac models are compatible with macOS Catalina:

• MacBook (Early 2015 or newer)
• MacBook Air (Mid 2012 or newer)
• MacBook Pro (Mid 2012 or newer)
• Mac mini (Late 2012 or newer)
• iMac (Late 2012 or newer)
• iMac Pro (2017)
• Mac Pro (Late 2013 or newer)

How to download and install macOS - Apple Support

https://apps.apple.com/gb/app/macos-catalina/id1466841314?ls=1&mt=12

And caution on 10.15 or higher, to see which Apps will no longer work in 10.15 get Go64...

https://www.stclairsoft.com/Go64/

For Mojave your drive will need to be partitioned to GUID. I just came from using Mavericks alone and mine was not so I had to re-partition the entire drive to meet the GUID requirement. Note that I was installing High Sierra. With a SSD you are probably better off using APFS but I don't know if you can have Mavericks (which can't understand APFS and probably wants Mac Journaled) and APFS on the same drive. Your alternative if you cannot make this work would be to check into one of the virtual machine software packages such as Parallels. Or just get an external drive and boot from that when you want to use the other OS.

https://discussions.apple.com/thread/8642161

You may want to double check APFS. I think one major problem you may encounter is Mavericks won't recognize an APFS drive -- they didn't exist when Mavericks was written. I seem to recall something about having to resort to really fancy procedures (booting to recovery partitions and then booting again?) when using dual boot Mavericks, SSDs, and OS versions that were of the generation you wish to use.