Teddy is now eating very well and using his litter tray as usual.
Daisy eats very well and is very active as a kitten should be. Tomorrow she's going to the vet for her vaccination.
They have been together and Daisy loves playing with Teddy. Sometimes she gets too excited and he leaves her however he has accepted the kitten very well.
I now feel much more confident that it was a good decision and I love when Daisy snuggles near me to sleep.
Thanks everyone
It probably all have already been said, but first I think working on the flea problem would be important. Take the kitten to a vet, if you already haven’t done so. They probably can’t give her any oral flea medication yet until she’s a certain weight, but they can comb and bathe her and wrap her to get rid of most of the fleas. This is what they did when I found my kitten at 2 months with fleas, too.
Then it would be best to vacuum the crap out of your house. Everywhere. Do it everyday for a while. And wash anything that you can in the washer. When you have fleas it’s important to get rid of them from the environment, otherwise they will just jump back onto the cats.
As for your other cat’s behavior I think it’s only been a week so it hasn’t been too long for adjustment. I’m sorry he got sick though. I hope that both your cat and kitten will grow towards better health and be able to adjust to each other’s presence. Cats can be so sensitive to changes it seems!
Edit: I just read your update! Glad things are going well!
After a difficult decision about the new name finally me and my son agreed with the name Peanut. He is so funny. I can't believe that I was so scared and feeling guilty when I got him. He is now part of the family and I can't imagine us without him.
Teddy has made a full recovery and finishes his medication tomorrow. He is back to his normal behaviour and playing a lot with Peanut. Also eating even more!!
As we are on holiday it's been easy to have them together most of the time as we can keep an eye when they are playing. Teddy is very gentle with Peanut but of course the run a lot in the house and play "fighting".
Flea treatment is getting better and almost all gone.
Thanks everyone for your replies as they have helped a lot.