I have 18 mbuna in my 60 gallon (which is essentially the smae as your 55 gallon, just an inch longer front to back) and it's a very fun and relatively peaceful set up.
It's gonna take some trial and error especially since you bought yours at petland where it is probable that 90% of the fish are hybrids and you don't know what you are getting yourself into.
I would suggest giving the fish a lot of rockwork for hiding spots and definitely over filter.
If you refuse to get another filter, I would not get any more fish... the 10 you already have might end up being to much for the one penguin 350.