I would like another cat and I feel the others would too but their biggest fear is that if we got another cat, they wouldn't get along. When I was young we had two cats at home and it didn't work out because our first cat should have been an only cat, he wanted to rule the house and didn't react well at all to a second one being brought in.
If we were to get Cole a companion, should we get him one about his age, older, younger?
I suspect the cat you had when growing up was older than 7 months when first introduced to your other cat, although correct me if I'm wrong. If Cole were much older, you might have an issue...but, at 7 months it would be unusual for him to be so "set in his ways" that he would need to be an only cat...not impossible, but unlikely.
I note that I introduced my girls when they were quite young (Muffs was 5 months, Abby was 10 weeks), and I had a rough introduction (3 months). That said, mine was an unusual situation and was not the norm. And, on the plus side, my girls are now best friends...so the problems weren't permanent. I've never regretted adopting Abby, despite the initial difficult introduction. My girls are now very close and you rarely see one without the other.
If you do get a companion, I would go with one that is at least 4 months old, but not much older than 1 year. I would try to match Cole's personality as best you can. I don't mean another cat that's high strung, jumps at every noise and bites!
But, if on the whole, Cole is more of a shy, quiet cat, then try to adopt another shy, quiet cat. If he tends to be more active and rambunctious, then go with another active cat...and so forth.
It would be also be best to adopt a cat that is accustomed to being with other cats and gets along with other cats (if you adopt from a rescue, the foster parents can likely advise you which cats do well with others).