Well , of course this is possible. But you need to do some extra work for it. In a generic case for any layout, You need to extend the WebcamSubscriber class and override the layoutStreams() function. It is in this function that the streams are laid out in a grid structure. You can have any layout you want. It will be more clear once you see the function in detail.
For your case there can be a simpler solution, if you know who are the users you want to show in small grid and who are the ones in larger grid.
In that case ,You can have a WebcamSubsriber that has all streams of all publisherIDs except the bigger ones(see my previous post how can specify users whose streams can be shown). For the bigger ones you can have a Subscriber each containing only that ID.
Then layout all these webcamsubscribers in a VBox accordingly. Look at the example zoomLayout, that shows one user with a big image in one webcamsubscriber and all other users below in another subscriber horizontally laid out.