Yeah OK. I always thought it was too bad they discontinued the DB-19 port on the Color Classic, for this exact reason, since it was the last of the compacts anyway. It's the only one that can't use an external floppy drive.
I presume you have the Apple 3.5" external (800K) on the SE (two internal floppys?), and the 1.44MB Superdrive external on the SE/30? Another idea for display would be to put an HD 20 under the 512K. Since a 512K looks exactly like a 128K from the front (actually everywhere except the rear case badge), the HD 20 would be a better way to set them apart as it was a quite common accessory, as well as for the early Mac Plus. Is your Plus Platinum, or beige? If Platinum, you might consider the HD SC 20 to put under it (the SC 20 also came in beige – the only new Apple product I am aware was sold simultaneously in both Platinum & beige). If beige, the Macintosh 800K external drive is the logical pairing, since that's what it was designed for, and or the 512K (especially if your Plus is Platinum and/or you have an HD 20, or if your SE has 2 floppy drives already).