I've successfully kept as many as six pairs of Convicts in a 30g, lol. Non stop babies and non stop action. No murders though. So, yeah, I think a pair would be fine. I currently have five cons in my 30g now, two males and three females. There's currently two groups of fry.
I'll probably get flamed for this, but I don't care. I take good care of my fishes. My first Convict breeding colony for a safe food supply for my big tank was 15 cons in a 20g. I had seven little flowerpots in there for caves. They made babies non stop too. No murders in that set up for a year and a half (I moved all those cons to the big tank). And yes, water was kept perfect with massive filtration overkill and frequent water changes.