I think a 55 gal, with your current fish stock, might be a bit sall for two I'm afraid. Especially if you're planning for it to be their long-term home. I have one in a 50 gal at the min with a few other larger fish (Ruby shark, plec, senegal birchir, 3 spot gourami, angel fish and syno petricola). and to be honest I can forsee the need to upgrade to a larger tank in the not too dim and distant future.
MY kisser was moved over from a 140 litre and you wouldn't believe how quickly she's grown since.
As said they can get to be pretty huge and I would imagine some of your other fish would make a nice meal for it when it starts getting big.
Overall, I would say that a kisser in a 55 gal isn't unreasonable but I think you would have to re-think your other stocking to make it a suitable home.
Also, what are the dimensions. In fairness I wouldn;t recomend less that a 2 foot width and 4 foot length. If you think that they can reach 12 inches you want to leave some room for them to turn around and generall move!