Will Swift 5.1 also be available on iOS 12 on MacOS Mojave?

Should be backwards compatible in that example, if that's what you're asking.

Or are you hoping to wedge it in somewhere else? Perhaps if you explained the why behind your ask...

>Is Swift 5.1 only available in MacOS Catalina/iOS 13 combo?

To answer that outright, ignoring your different title question (and the Xcode 11/iOS 13 beta that runs on latest Mojave), yes.

> if we can compile Swift 5.1 on Mojave

>can we still move the app to Swift 5.1?

You can't submit iOS 13/Swift 5.1 apps to the store at this time, so no - any questions in that regard are moot - you can't move your store apps yet. You'll know when it's time.

When it's time, again, expect it to be backwards compatible, within reason, and not a risk to customers on 'older' versions, those versions defined as still supported by current tools, iOSs and the store, at the time of public release.

As for experimenting, that's what the betas are for.

But it sounds like betas are new to you, so my advice at this time is to air gap them for anything but -early- look-ahead testing of your own apps. They are only days old, so you can relax about any immediate impact on users.