Since you want to adopt another cat anyway, I think the idea of adopting a friendly second cat and giving Mille a chance to see how she responds to the new addition before giving her up is a good idea. I would advise that you adopt a kitten, since they are unconditionally loving and accepting, and it is quite possible that Mille will adopt the kitten and assume a motherly role. From my experience, resident cats often feel protective towards a new young scared kitten. There may be a few days of hissing, but that usually passes quite quickly. If the shelter you have been dealing with does not have any kittens or indoor only cats, can you look elsewhere? In your local paper? Another shelter? It is so important to pick just the right Kitty this time around instead of letting the shelter talk you into another cat that will not be the affectionate, content kitty that you are looking for. Also, if you get a kitten as opposed to an older cat, you won't have to worry about frequent vet bills after spay and initial vaccines. Older cats do require more veterinary care. Adding a kitten as a second cat won't cost much more than having one cat. Anyway, this is just more food for thought for you.