Well, I am a big believer in the idea that most dogs really enjoy the company of other dogs. If your female is to be left home alone much, it might be very nice for her to have a companion. If spaniels are at all like setters, they will enjoy a buddy.
Yes, the opposite sex is a great idea, although two of the same will usually get along too. Two females tend to have more troubles than two males, but not always.
As far as the ideal spacing in age, I think you would be fine for the new one to arrive when your dog is 8 months. She will be grown up enough to be house trained and have her basic training well along. She'll also be young enough to still have plenty of "play" in her to keep the little one busy.
The only advice I'd give would be to continue to make sure your female gets some time with you all on her own for training and such each day. It does not take a lot of time, even 15 minutes of one on one interaction with you, will keep her reassured that she has not been displaced by the newcomer.
We have two setters. Our male, Josey is just over two years old, and the female, Tessa is 1 year. So Josey was a little over a year old when Tessa came along and that worked out great. If you want to see how that transition went, you can look at Tessa's puppy album here. The picture order is backwards, so if you want to see it in order start at the end and push "previous"
http://www.dogforum.com/members/3356...uppy-pics-283/
Well, best of luck. Hope you post pics when your new puppy arrives!