I'm new to Step Up Transformers

is this version yours?

or this earlier version?

In either case, you would use the MC input, and mess with your load options, try 100, try 500, hear any differences, preference, you cannot hurt anything

An MI cartridge with a 10 ohm internal resistance like the Paua and some other soundsmith MIs is not comparable to an LOMC with the same internal R, because LOMI cartridges have a MUCH higher inductance, at least 100X to 1000X higher than the LOMC. The inductance adds to the resistance as frequently goes up. Thus the recommendation for a 470 ohm load as a minimum. Dave Slagle mentioned yet another reason for the recommendation. I run MI cartridges with a 1k ohm load, minimum, usually 47K ohms.

The Zestos model I have is the PS-1 (bottom photo).  I did experiment when I got the cart (2 weeks ago) and found 1000 ohms to be too bright and thin. 

   I believe I ended up at 800 ohms (at least for now) and that setting gave a nice airy high end without it getting thin. 

    Lower settings started getting too dark or dull.  But I am told that carts have a break in period so all that could change.

FWIW, my favorite cartridge is a SoundSmith Hyperion, my SUTs are 10:1, but I also fearlessly interchange a variety of LOMC from Ortofon, AT (the recently introduced AT20 is quite nice), and others.  My philosophy is to try things and listen.  BTW, listen means for a while, not just for a minute or two, maybe at least an hour or more after making a change.  I like to give my ears and mind a chance to adjust to the sound of what I am trying.

I’d say wait through 2-3 listening sessions at least one hour each in length, before making any judgement of a new cartridge, and that’s only if you’re very familiar on a long term basis with the phono stage. And even then, cartridges do change over time from new, probably more so than any other piece of gear.