Hi,
My 8 mth old cocker is actually a rescue cocker. I don't think I would take on another dog until he is at least 18 months old. We already have an older dog too, Millie is almost 4. We would like a third dog but we have decided to wait until next year. Toby is at training classes at the moment and is going through a hyperactive chew-everything stage which is very demanding.
My mum took on two rescue dogs within a month of each other, one is 8 mths and the other is 7 mths, and I have to say, much as she adores them, she is going through hell with them at the moment, they bound around her house playfighting ALL DAY, knocking things flying, they both chew things up and egg each other on all the time. When she just had the first dog he was well-behaved but since introducing the second dog he has started to be very "naughty".
Like Shirley I think it's great you have considered taking on an older dog. But you might be a bit stretched attention-wise if that dog has problems to deal with and your current dog is still only a baby.
Good luck whatever you decide