First up I add another 3-4 glass fish, the like schools from what I hear, 2nd what sex are the swordtail ideal ratio is 1m/2-3 f and third yes you could have a pair of keyholes
If you happen to get a male and a female and they spawn, you'll probably run into some aggression problems if they spawn.
If you get 2 males, as they grow/age they might become less friendly towards each other.. My 2 keyholes were fine for about a year.. 1 grew larger then the other and now attacks the smaller one a lot..
When i had my 2 keyholes they found my neons more prey than friends. As soon as they went in they were trying to kill them. Just a bit of advice if you are going to mix them
Seems like swings and roundabouts mate. IMO keyholes are just about the most peaceful cichlid out there, even when spawning they'll chase but do no damage in my experience. I'd say go for it. How you'll find a pair is more tricky. They're impossible to sex at the size they're usually seen for sale at. Unless you can find a sexed pair maybe buy a group of 6 and let them pair off naturally...