"Ken, could you elaborate on what you think the effective range of each strop material is with regard to paste particle size?"
This is actually a tough question. I would have to divide it up into effective and optimal.
paper - depends on the paper. Finer paper works best for finer grits. You can use paper plain as it does have abrasive content, primarily from the clay content of the water used to refine it The pulp itself also has natural abrasives. Check the screens in your faucet if you think your water has no particulates in it
For coarse grits paper tears up easily, so Balsa works good here. Real coarse and the grit won't stick to the paper - eg 80, 200 and 300 micron CBN. Copy paper used for printing is cheap and OK for mud samples in coarse to medium fine grits. For fine paper I like Bagasse - made from sugar cane pulp. Very smooth but a bit too thin. Relatively cheap. My favorite paper comes from Rhodia / Clairfontaine - durable and silky smooth. Even their card stock works well. There are also several Japanese papers that work well. I don't really notice a difference between Japanese and American newspapers. BTW Murray will sometimes strop on newsprint. His edges CAN be improved with a 10k Glassstone FWIW, but he does squeeze out excellent performance from a 6k King. Some say the Japanese ink is best (I think this is BS) and makes Japanese newsprint better. Even glossy paper has abrasive characteristics. Strop on a brochure. The downside of paper is durability for compounds - the compound outlasts the paper. The upside is flexibility. You can back it with soft backing for convex edges or hard backing for precise flat bevels. Or use the mud or compounds on it like a fingerstone.
I use Balsa for stone mud preparations, especially natural stone muds like aotos. It's pretty versatile. For coarse muds or CBN, the particles embed into the balsa and you don't get the full height of the particle effectively reducing the grit size a bit. Much more durable than paper. I have used it over a wide grit range but at the finer grits .25 and finer it begins to interfere with the abrasive. Not a great strop on its own without abrasive.
Horse is a bit finer and harder than cow and Kangaroo is finest by far. All have their own natural abrasive and surface texture. Kangaroo is thinnest and therefore gives the most precise edges. Cow is most widely available and cheapest. I use it for belts and strops. Belts have notoriously bad 'joins' so it gives an annoying 'bump' each time it goes around. The belts also stretch eventually becoming unusable. These days I prefer linen belts.
Kangaroo can be used with compounds as fine as 640,000 grit or as coarse as 2 microns. Horse is OK up to .25 but less so after that. Cow is a similar range to horse.
Linen - I use this with CBN and diamond as coarse as 80 microns and as fine as .25. I think it has a coarseness in it's own right so probably below 1 micron it is not optimal. A very durable substrate. I have linen belts for all the major grinder sizes from 2x72 inch down to all the Worksharp belt sizes - eg 1x30 and 1x42, etc.
Felt - I rarely use felt for much of anything. Some like it.
Nanocloth - Surprisingly most think of this for just the ultrafine grits and here it has no peer. You can use this well up to my 1.2 million grit polycrystalline diamond product. BUT you can also use it down to 80 and even 200 and 300 micron grit! It is THE most versatile substrate out there bar none. Works GRAT for natural stone slurries, etc etc. I consider it the king of substrates. Far more durable than paper. Holds a denser coat of abrasives than leather of all sorts. Superb with CBN, Boron Carbide sprays and emulsions.
Stones - the forgotten substrate. I like to match synthetic stone grits with compounds of comparable grit. Particularly effective for the coarsest grits. So from 300 microns to half micron (30k). For natural stones, I go with matching grits in the mid grit and coarse stones and with finer grits than the stone itself on finer grit stones (awasedo) So from 80 micron (Ohmura) to 0.025 (Ozuku, Nakayama).
This whole posting is a very rough or cursory overview. More detail would be a whole book.