Nah,Cichlid aggression is overrated,look at my tank pics thread and youll see my tank is always jam packed with Cichlids
Trust me Kribs will hold there own,mine are currently breeding and my 2" male Krib trys to beat up my 9" Male Severum! None of my stock pick fights,they just squabble noq and then...aslong as you put in some caves of some sort youll have no bother.
All the cichlids you listed are tame,Firemouths can have there moments,but trust me they are all bark and no bite,they love a squabble but tend to swim off rather than actually fight.
Fm's are alot like Convicts but nowhere near as agressive.
Convicts are one of my fave fish but everytime I have tried to have them they end up beating something up so i gave up with them,they seem to HATE Fms.
I think the rule of the thumb with cichlid crowding/communitys is to pick all semi agressive specimens...no aggressive ie: convicts and no placid ie: Rams,Keyholes.
I do the same in my marine tank also...everyone told me not to,but it works brilliantly i stocked Clarkii Clown,Strawberry Dottyback,Humbug Damsel,Azure Damsel,Yellow tang...all semi aggressive fish,makes for a more interesting tank.