Good morning,

I have the RB5009 PoE version.
It is powered by its dedicted power supplier that feeds 48V into the router

I would like to add a switch and looking at the RB260GSP I can see that it needs 8V - 30V

It seems that these two units can not work together, right?

Thanks

Not with a 48V power supply.
They may work if you power the RB5009 at a lower voltage 24V or so.
But if you do so, all Poe out ports of the RB5009 will be at this lower voltage and other devices actually 802.3af/at compatibile
that you may want to connect to other ports won't work.
There are splitters/converters that can be used in case of need, but if possible it is much better, particularly for new installations, to "choose" one of the two, either passive 24V or 48V 802.3af/at for the whole network.

They can work together - You just can't use the RB5009 to power the RB260SP.

If You can work with passive PoE at 24V, then the RB260SP can power the RB5009 - I'm doing it at home.