Should the inside of the slot where the guide pins go be cleaned out and filled with a bit of silicone paste when re-lubing the guide pins? Or does lubing the guide pins do enough.
Yes. It is important for the pins to be clean and lubed properly so the caliper can move freely. Otherwise the pads can wear unevenly. So, if there is crud in the pin hole, clean it out. There should be rubber seals to keep dirt out. Those may need to be replaced if they are worn.
Make sure to use high temperature grease made for brakes. The calipers get very hot and the grease needs to withstand it.
Should the inside of the slot where the guide pins go be cleaned out?
Yes, especially if it hasn't been maintained, or the wrong/different lubricant has been used. I like to spray brake cleaner and use a small narrow cylindrical shape brush to clean it out and remove any existing lubricant. The brake clean will evaporate quickly so it won't contaminate or dilute the grease.
Should the inside of the slot be filled with silicone paste?
No, if you do this, you risk getting grease at the end of the slot and on the tip of the pin. This can cause hydro-lock and the pin won't be able to move all the way down the slot as the lubricant won't compress. Then you will have a similar situation to a seized pin.
Does lubing the guide pins do enough?
Yes, use a silicone or ceramic lubricant made for brake applications, don't over-do it, don't put any on the tip of the pin (to avoid hydo-lock as described above).