Although I've built my line as R3 minimum, some of my locos have ventured onto R1 from time to time for indoor testing or when visiting friends' lines. In my experience:
The Consolidation has trouble due to the long wheelbase (even though the middle drivers are flangeless) and the tender fall-plate gets in the way, causing derailments.
Mallett - can cope when running light as both drive units are articulated, but results in a lot of coupling overhang which can cause problems with the stock.
3 truck Shay - copes ok, but I think you need to be careful which of the two holes you use to couple the 3rd unit, otherwise it causes problems with the driveshaft linkage.
Big Hauler 4-6-0 - despite being sold in sets with R1 curves can struggle a little and can derail the leading pilot.
Baldwin 2-4-2T - copes ok.