Yes, thin 3/16-1/4" plywood should be fine.
Depends on the floorcovering - In ours, the
marmoleum is run under the cabinets just like it was done originally - makes for a clean "bottom shelf". I wouldn't run carpet or thick wood/laminate floors under them though. Carpet would be too tough to replace later and wood floors would add too much weight.
Shari
