First he needs the ring generator if he wants selective ringing on a 551. What I would do is make two spare buttons on a 6btn to ring the two stations if they are free. Kinda like a speed dial.

Keep the Tone commander for the SLT. Like Ed said you can Have intercom ring or intercom buzz. But if you choose to have Intercom Ring and selective ringing You will find it much easier to use the ring generator as the input to the rt1900. The common audible input would be much more of a hassle to achieve both intercom ring and common audible. Just my thought's but I would like to know a bit more.. I like the combination of using both the ring generator and common audible idea. Yes you can send the the ringing though the RT for intercom ring

But don't try to take RCRG on a common and send it to the rt 1900 for intercom. This is when the ring generator is needed.