Yes it is possible. I would do 4 <This is because, let's say that 4 of the Keyhole cichlids pair up; one will be left and most likely picked on or bullied by the other pairs> instead of 5. Since you will have more than one in there, be prepared if they pair up (Make sure to have a back-up plan).
Also may I recommend that you add schooling fish that are big enough that they won't be eaten by the Keyholes.
Not necessarily, I've seen their behavior before, particularly keyholes (was completely amazed). If I am able to pair them, and if they do want to breed, I'd very much love to see that, as I know it can be beautiful to see. Again if it will only cause problems then I have no issue leaving it to another time in the future, but as you will know (as so many people do), I want to get the most personality from my fish, making the interaction something to be proud of, as well as seeing something new.