You need the sidecar tyre to "slide" so that the traction is on the rear tyre & then thats were the passengers job comes in.
Wide tyres on the chair wheels are more of a hinderance, you dont want the full width on the track otherwise the 2 of you get spat out. This is why chassis makers have shied away from putting suspension on the sidecar wheel. Perfect that suspension & you can run a skinny tyre which means less drag in a straigfht line which equals more speed.
Long outfits( suspension working in an arc) should run anti roll bars which run from front to back.
Back to your original question Ian, always go for a high pressure, not running a classic I'm no good to you
