Sorry Vindi2 - I feel like I steered you down the wrong path with the Leviton switches. I had no idea you already had that many X10 slave switches installed. I recommended the Leviton because they happen to be what I'm familiar with. I would doubt that the RWS19 is compatible with the Leviton - maybe one of the other forum members has direct experience.
I am not familiar with the X10 brand switches in a 3 way (or more configuration). I did a quick search of the X10 site and couldn't find a 1000 watt dimmer. Maybe something in the X10Pro line (forum members - some more help)?
Another option would be a 10amp relay switch - again I'm not sure that an X10 relay switch is compatible with your existing RWS19 Slave switches.
Edit:
Another option would be a 10amp relay switch. I forgot about surge current with incandescents - Anyone know what incandescent rating of a 10 (or 15A) X10 brand switch?