Four adult firemouths will need all the space your 55g can provide. Adding kribs to the mix will only lead to the demise of the kribs at some point.
At the LFS, therefore, I would instead concentrate on adding the dithers, catfish, etc. that will go with your firemouths. Look at rainbows, synodontis, and interesting plecos to fill out your tank population. See what's available at the LFS, and then do some research to select the most appropriate tankmates for your cichlids.
I agree, the kribs would be toast if the Firemouths ever spawned. Two species of bottom dwelling cichlids is a bad idea in anything other than a 100 gallon. There would be simply too many confrontations that would eventually end in someones death.
thanks sorry i left this post so late at the lfs i bought 1 royal plec which set me back a bit thanks alot i didnt want to go of bying fish that will end up eating each other.
Goodluck with the Royal Pleco, they are very nice fish.
Be aware that water quality is a must with these fish especially when young, they are considered somewhat fragile. Also bogwood is a must as these Plecs need wood in their diet to maintain normal digestion.
I recommend researching them at PlecoFanatics or Planetcatfish so as to provide for his needs as best as possible.