Folding Bikes - 20" x 1.75 rim. How low can you go?

Given the small wheel size, it seems like a question for the folding forums. I'm preparing to replace the tires on my Tote-Cycle. I currently have 406, 20 x 1.75 tires on what appears to be about a 32 mm rim. I'm looking to replace the rim, but hopefully keep the same tire size. There's lots of 406 rims out there, but most run a fair bit narrower than 32mm. How narrow can go and still use the 1.75 tires? I was looking at the Velocity Taipan which, at least from some vendors, is listed as a 20 x 1.75 rim with a width of 28 mm. Meanwhile, the Sun Rhyno Lite is listed as a 20 x 1.5 rim at 27.5 mm. So how narrow is too narrow for a 1.75 rim? I'm considering both of those rims, but also considering that might have to spring for a more expensive, wider rim. But then the tires I'm thinking about are Schwalbe Marathon Plus tires, which I believe I've heard can run a little smaller than advertised, so maybe a slightly narrow tire is fine.

So what's a good rim width? And does anyone have any experience with any of those rims or using the 20" Marathons?

I recently changed tires, from the 20 X 1.5 Marathon Racer to the 20 X 1.75 Marathon Plus. I wanted a very puncture resistant tire, and I like the ride. The Plus is a much heavier tire.
I have 17mm wide rims, and I was able to keep them. I am right on the borderline. If I were buying new rims, I would go with a 21mm wide.

Thanks, that's great info. I am a little worried about the weight of Marathon Plus, but it's an old, steel bike, and I'm switching from steel rims to an alloy, so I'm thinking I will still come out ahead. Plus between the axle-mounted fenders and front rack and the internal hub, I'm willing to take on a heavier tire if it means means not having to disconnect all that crap to change a tire. Knowing that I could go as low as 21 (or 17) mm really opens up my rim options. Thanks again.