I'm sorry about the problems :(
They have always adviced me to have as many litter boxes as I have cats, and it worked for me.
When I brought home cat no. 3 (which is a stray I took in) I didn't immediately have an extra litterbox and one of them, I *think* the latest addition, peed on my bedroom floor all the time. But once I had another litterbox AND had all 3 of them cleaned out properly, the problem disappeared.
Maybe you could keep the kitten in a seperate room or in a bench for a couple of days untill they have gotten more used to eachothers smell, then start to feed them, each on their own side of the door/benchwall to make them realise that they both will still be fed and then take it from there.
I'm sure that if you'll do a search on google, you'll find lots of other advice on how to introduce a new cat into your existing family.
And: my vet told me when I was getting worried about the cats never learning to get alone that it could take up to 2 to 3 months before they'll tolerate one another and I think it's true: mine still don't love eachother and they'll never sleep all bunched up together but they can co exist in the house just fine, eat their meals right next to eachother and they'll all share my lap or the sofa with me without too much trouble.
So please be patient for a while longer; this will take a little time.
I know this isn't an immediate solution for you but I hope you'll hang in there