“fullscreen” cannot be used as a condition in windows rules AFAIK, but you can achieve this with a simple kwin script (the syntax is very easy, nothing to be scared with). You can find on the KDE website a tutorial and the whole API set.
Let me show you: We are going to add an event listener listening to any client going fullscreen (or de-fullscreening), and then we will apply the “keep below” property to the fullscreen ones.
you first need to add a rule to listen to clients changing fullscreen status: (references)

workspace.clientAdded.connect(client => {
    client.fullScreenChanged.connect(function() {fullscreenChanged(client)})
});

This will call the function fullscreenChanged each time a new client changes its fullscreen status, passing to the function the client. ( I don’t know why but workspace.clientFullScreenSet does not work, it would have been better, but whatever)

We need now to write the fullscreenChanged function. It should change the KeepBelow status, evaluating if the client has been fullscreened or de-fullscreened:

function fullscreenChanged(client){
    console.warn(client, client.fullScreen) // not needed, just to debug 
    if(client.fullScreen){
        // set as keep below
        client.keepBelow = true;
    } else {
        // unset as keep below
        client.keepBelow = false;
    }
}

so the final script will be:

function fullscreenChanged(client){
    console.warn(client, client.fullScreen) // not needed, just to debug 
    if(client.fullScreen){
        // set as keep below
        client.keepBelow = true;
    } else {
        // unset as keep below
        client.keepBelow = false;
    }
}

workspace.clientAdded.connect(client => {
    client.fullScreenChanged.connect(function() {fullscreenChanged(client)})
});

you can follow the instruction on the tutorial on how to install it. Hope this helps!
(I haven’t tested it much, in case of any problem feel free to ask!)