A 12v solenoid can be run @ 24v as long as it's only for a few 10's of seconds.

The pushing ability is limited by the mass ( weight ) of your robot on the wheels and the co-efficient of friction of the tyres.

The BEST you could possibly achieve is a pushing force in kg's equal to that of your robot.... 0.9 times the bots weight in kg's is probably closer to the max you could achieve
and 4 driven wheels gives this possibility... all other configurations have a percentage of the bots mass sitting on castors or skids thus it's not available to be used for grip.