I'm afraid i can't help with how many you can keep in that cage as i only have the one myself!
But i can tell you that canaries are happy to live with other birds so long as they have other canaries around them. My grandfather has bred and shown canaries for many years and his lived with Cockatiels in an outdoor aviary without any trouble I've also heard that zebra finches and budgies get on well with canaries.
In a cage that size you shouldn't have any trouble with keeping more than one cock but i'd definately make sure i had more hens. 3-4 cocks is said to be a more peaceful number than 2!

HTH, i'm sure your new chap is very grateful for your efforts