Spack, it might be easier if you look at it backwards. Start at the switch/dimmer. The switch will require an X-10 signal to operate. It doesn't care where that comes from. It can come from a Maxi-controller, an X-10 handheld keypad/transceiver setup, an IR543 or a computer. What you add as a source depends on what you want for your convenience.

A Maxi-controller generates X-10 and you would need nothing further to operate the switch if that's where you wanted to stop.

The IR543 converts an IR signal from a remote to an X-10 command and sends it to the switch. Therefore, you don't need a Maxi-controller in use to operate the switch in that configuration. However, with most scene capable switches I am aware of, you need the Maxi-controller to initially program the switch, or, perhaps your computer if you have that set up.

I'm not familiar with the Harmony remote, but if it's programmable, I should think you would use it to send an IR signal to your IR543, which would convert the signal to X-10 and send it on to the switch. I've also never actually used an IR543, so if I have this wrong, someone can jump in with a correction.