Yes they are. I have always wired 4 post solenoids the way I have seen Ford do it with the battery connected to the large post near the "S" terminal, & it works that way. Thinking about it, it does seems like it would work the other way too. I have never tried reversing them.
Internally when power is applied to the "S" terminal, contacts close & both large terminals & the small "I" terminal all are connected together. It seems like it must be separate contacts inside for each of the output terminals, otherwise voltage from the coil would backfeed thru the "I" terminal, thru the solenoid, & try to turn the starter anytime there is voltage to the coil. If that thinking is correct, it seems like the solenoid would work with the large posts connected either way. Now you have my curiosity up. Next time I have some time with a solenoid, I will fool around with it & see if that is correct.
But since I haven't actually tried it reversed, I went with what I do know works in my experience.
However you wire it, for a mini starter, be sure that all other wires feeding power elsewhere attach to the same side as the cable from the battery, & the large cable to the starter is on the same side as the battery cable, while the small wire to the starter is connected to the opposite side of the battery cable.