Will a "on-off-on" and "on-on-on" toggle switch ( both DPDT with 6 terminals ) do the same thing in the sense of being able to switch between these on the P-RAIL?
No. On-Off-On is used because it works by shorting out one or the other coil. If you use an On-On-On, the middle position, both pickups are shorted (also with a two pole On-On-On switch, the middle position is special).
Does the voltage and amperage values of a switch make any difference when used in guitars?
No, not at all.