And what kind of underwear should each race have?
The Khajiit and Argonians would have to have garments that account for their tails.
The tail extends from the spine so I'd imagine they'd have some kind of tie or button to fasten around the top of the tail. For the Khajiit at least I can't imagine pushing a tail backwards against the flow of the fur through a hole would be very comfortable, though I guess since it's underwear anyway the hole could be sufficiently large since clothes are worn over underwear anyway. For the Argonians their tails are so thick they'd need some kind of fastener to keep the back sealed up well.
This extends to their normal garments too. Some kind of belt-buckle like flap or button could account for tails in normal garments. I highly, highly doubt Bethesda is going that far to make the clothes realistic though. There will likely be the standard "the tail just goes right through the cloths" design, implying some kind of tight fit design for the clothes without getting into the details.