Welcome to Tropical Fish Keeping.
Convict cichlids can be aggressive with other fish, and the kribs may be too close a relative (also a cichlid). Certainly keeping more than one species of cichlid in a 20g (80 litre) aquarium is not recommended. So, adding a Firemouth and Gold Severum (more cichlids) in so small a space is bound to cause problems. Severums grow to 8 inches which is way too large for a 20g even when they are juveniles; they need water space/quality to grow healthily. They may appear to get along fine now, but this is sure to change as the fish settle in and their instincts and behaviours develop.
I believe BN plecos need wood to rasp. Once cured (soaked, boiled) wood will not discolour water. If you want to keep plecos I recommend a couple pieces of wood.
I am not a cichlid expert, so until one of our members who is comes along, my suggestion is to decide on which small cichlid you want, and acquire a pair (male/female). A pair of convicts would be fine in a 20g, with a pleco. Or a pair of Firemouths, but this may be pushing it [I'll let the experts comment]. Or just a pair of kribs.
Last comment on cycling, I don't know how you managed this in only a week, but there is good info on cycling in this article that is a sticky at the head of the Freshwater Aquariums section:
And a companion article on fish stocking may be helpful: