Recumbent - gear inches and low gear

Hi all
I have a trek r200. The low gearing is not low enough. I've learned a whole new bunch of terms and have calculated gear-inches of 26.6, plus I have at present oversize tires on my 20 inch rims. I have a single chain ring of 40 teeth that runs to a 5 speed derail of 24 teeth largest cog that is on a fixed 24 tooth cog that drives the rear 8 speed derail (30-11). It seems the easiest way to get some lower gearing would be to change the fixed 24 tooth to a smaller cog size, but of course that would also change my high end. So then I'd need to change the 5 speed 24 to a larger cog size etc. So two questions:
How large a jump can a derailer accommodate between cogs. I've read 10 to up to 16 percent.
Next question that I know is not possible to get a definitive answer to, but what do other bent riders feel is a low enough gear ratio for touring. I manage to go up some fairly steep hills, but my knees are killing me and I guess that's not a good thing. I've read that a good cadence of 80 is better in the long run. Any thoughts out there...thanks

If you only want a lower low without losing your high gear, you could do a couple of things. First, you could replace your rear cassette with an 11-32, which would make your low gear just under 25 inches. Next, if you can breake into the mid-drive, you can try to change the 24-tooth that goes to the front chain to something bigger. To answer your question, a 3-tooth difference shifts fine, 4 teeth still shifts OK, after that shifting starts getting slower. There is no line in the sand, it just depends on your tolerance of possible slow shifting. You might want to replace several cogs so you get a nice progression of gears. Changing either the front chainring or the fixed gear on the mid-drive will give you a lower low but at the expense of your high gear.

For loaded touring, I've heard numbers all the way down to 15 gear inches, although I think it'd be hard to balance at the speeds that would allow.

I'll probably buy a shimano cassette and pull it apart for the cogs, which makes me ask another question.How come cassettes come apart thus leading one to believe that separate cogs can be purchased. I haven't seen any for sale anywhere online.....? good illusion maybe?

When cassettes first came out, they used discrete cogs. Then the mfgs discovered that they could make the cassette assembly lighter by mounting the biggest cogs on a lightweight carrier. At first it was just the biggest 3 cogs, then 4, nowadays it's all but the smallest 2 or 3. You can still get cogs if you search around, at worst by buying another cassette for parts; and you can often unrivet the carriers and change the gear ratios. I think Sheldon Brown has written some on the subject.