Hi KateO -- another question I have is...do you have enclosed litter boxes? The reason I ask is that I have three total boxes for two cats so they don't feel "trapped" but two of them are enclosed...one of them is a dome but they can see out of it. When I spoke to my vet about the litter box situation (which was -- knock on wood -- never a problem) he said if one cat goes to the more open one, they wanted to keep an *eye* on the other...lo and behold, Cassie was the one who frequented the open dome while Napoleon would go anywhere LOL (in a box, though, never on the floor or rug or anything). So definitely consider that...
If not, then Hissy's suggestion is right on - Tails may be trying to tell you something is wrong, so def don't rule out a vet visit.
My other suggestion is more behavioral...my cats were havnig a tough time coexisting (still do, but it's only been two months) so I started to play interactively with them. It's been bearable (though not ideal, there are still some growls and hisses from each camp every now and then but they seem to deal more with the idea that the *other* is there) so I would highly suggest doing that to get Tails more comfortable...sounds like a territory issue. As long as those kits are there -- overtly threatening or not -- Tails will view them as a threat. There's no reason to get rid of Tails
you just need to encourage her. Good luck