Amber,

As long as you introduce them gradually, I don't see a problem at all! I have 8 cats and they all get along for the most part.

The key is to rub your 10 month old cat down with a towel and let the new kitty smell his scent and vice versa. Keep the new kitty in a separate room for about a week. Then gradually let the new kitty out to explore SUPERVISED. There will be hissing, growling and spitting, but as long as there's no blood shed, don't worry. And don't expect it to take overnight, patience is the key.

Good luck and keep us posted.