1) Whether any particular USB Floppy will work for Windows installation depends on your motherboard BIOS emulation, if it does trick Windows into thinking it's a floppy drive.
2) If your bios can do #1, you might try a USB thumbdrive, as there's no reason to use a USB floppy if you don't have to. With bios that can emulate a USB floppy sufficient to fool windows, there is a reasonable chance it uses same emulation with another device, BUT some bios that even could, might have that setting hidden. You'd just have to try it and see...
3) Boot a CDR or DVD to flash the bios if all else fails, or if you have a FAT16 or 32 hard drive, whatever you have that will boot to DOS will flash the bios fine. Frankly, using a USB floppy to flash a bios seems a bad thing to do since your board had to be able to boot USB already, and among all possible USB devices you're using the one device with frail media susceptible to failure far more often than any other type. Granted, usually it work work fine and you won't have a floppy disk failure, but MOST ways to flash work fine too- we're talking about that minor chance of a problem and how to make that even less likely.
4) For emergency BIOS recovery, like when your board bios is unworkable and all you can do is hope the bootblock seeks a floppy, in that special recovery mode the vast majority of motherboards cannot boot a USB floppy, ONLY a regular floppy as the Super I/O chip handles that natively without further bios code necessary.
5) Flashing from Windows is the most risky, you add another layer of complication, something that can go wrong. This is contrasted to flashing from DOS with a reliable media type as mentioned above, not from a floppy. Thus, booting to a flash memory based device to DOS to flash the bios is the best way if/when your board can do it.