Welcome Patx2. Let me say I am so sorry for your loss of Coco. There are so many of us who have recently had to say goodbye to an angel on this site. I agree. It is the most difficult thing we have to do. They can't tell us when they are ready. We can't let them suffer. You did the right thing. There is no cure for FIP.
little Coco
Find my Brownie and play. He loved playing.
Now - on to happier things. One of the biggest mistakes we can make when introducing cats is to think we can put them together right away and everything will be wonderful. It isn't. Especially when one of the gang is grieving. The introductions need to be done slowly and correctly in order to ensure a happy, healthy household. Btw - six weeks is a little young to be taken from momma cat. In order to be socialized properly by mom it should be at least 8 to 10 weeks. Optimally 12. Mom is busy till that time teaching her brood how to play nice, and fair. She is most likely still nursing as well. At the moment I have a mom and six kittens who are just over three months and are still nursing. I'm sure at this point it's more of a comfort thing because mom is almost dried up but still, they love it.
One of the best sites I have found is:
http://www.squidoo.com/multi-cat-management
It is very well written by a woman who walks the walk every day. You will enjoy reading it and learning. I know there will be other cat enthusiasts along soon with more info.
Again - welcome.