Matching the cartridge to the phono stage

You mentioned Benz and Soundsmith.  Many Benz LOMC cartridges have high-ish internal resistance, like the Hana SL (33 ohms).  So those are just as difficult to match with your phono stage as is the SL.  Soundsmith make some "Low Output Moving Iron" cartridges.  Those also would not be an ideal match for your phono stage, even though their internal resistance is typically only 10 ohms (see below).  If you must have a LOMC cartridge, stick with those that have a low-ish internal resistance.  Like less than 12 ohms, for example, AND a low inductance (that's why the Soundsmith LOMI cartridges need an input impedance of greater than 100 ohms; their inductance is much much higher than that of a typical LOMC.)

As for your last point about how the preamp design deals with the RFI, isn't some level of variability in load resistance needed? The market for MC carts presents a lot of options, all of which have very different specs. Or am I not understanding your point?

The short answer is 'no'. People use the loading as a tone control with many preamps, on account of without it the preamp sounds bright. This is usually due to distortion as I mentioned prior. Get rid of the RFI and the distortion goes away.

So if your phono section is properly designed, you'll find the loading makes little difference. That remaining difference will be because the cantilever is less supple tracing the groove when the cartridge is loaded. The specs the cartridge manufacturer posts about compliance are usually when the cartridge is driving a 47KOhm load, not 100 Ohms or the like.

Put another way, if the phono section is properly designed its plug and play.

You mentioned Benz and Soundsmith.  Many Benz LOMC cartridges have high-ish internal resistance, like the Hana SL (33 ohms).  So those are just as difficult to match with your phono stage as is the SL.

Specifically, these Benz models are: Gullwing, Ruby, and LPS. They share a ruby plate armature which significantly lowers output level relative to any given coil arrangement. They're a tough match to a SUT and OP's phono stage. Typical for these is 0.35V @ 40 ohms or 0.7mV @ 80 ohms.

The lower Benz lines use an iron cross armature that is much more efficient. These work great with the right SUT, and should also be workable with OP's phono. The models include: Ace, Gilder, Wood, Zebrawood, Reference, Ebony. Typical for these models is 0.4mV @ 12 ohms or 0.26mV @ 5 ohms. 

I appreciate all the excellent answers but the guy is new to this. A suggestion for an easy solution would be the right way one might think. How about a MM cartridge and his phono stage is all good. Ortofon MC carts are a good answer as well. Not sure about the Rega though. Some cartridges need spacers. What beginner wants to deal with that?

I wouldn't deal with spacers even after 45 years in the hobby, or rather ESPECIALLY after 45 years, because it is a "problem" so easily avoided.