My halfbeaks became dinner for my knights so i would be careful mixing the two.
That would depend on the species. Nomorhamphus ebrardtii, the red-fin halfbeak, is about 10 cm long and should be fine with knights. It's a brackish or hard water species, so will do well at SG 1.003. The problem will be finding them; they aren't rare, but they're not often sold as such. You have to know what you're looking for, and pick them out of the invariably mixed species Celebes halfbeak batches. That said, common Celebes halfbeaks can be kept in brackish water. They don't like it, and they won't breed very readily, but I've seen it done and they are prefectly healthy.
Sabby -- I'd skip the BBGs, and think about African clay gobies (Dormitator lebretonis), crazy fish (Butis butis), and candy-stripe gobies (Awaous flavus). All these are sold fairly widely, but you will need to hunt for them. Candy-stripes especially are lovely fish, but short-lived, so be sure and buy them as babies (~4 cm or so) rather than the very beautiful 10 cm adults, which will be dead within a year. African clay gobies are best in groups; though they squabble endlessly, they're harmless and its more like tiger barb playing that anything else. Though prdatory, they're otherwise nice fish, and swim in midwater, which is odd for gobies.
What about adding some more chromides? They're great fun for breeding. The wild type are especially rewarding because you get to see the fish in breeding condition change colour completely. The artificial all-orange form may be prettier, but you don't get to see this, so in some ways you lose out.
I'd also have a think about livebearers, look goodeids. Ameca splendens is a feisty little fish, but in a semi-robust community should be okay. Wild-type sailfin mollies are also very nice fish, if you can find them. In a 55 gallon tank, you should be able to keep two males without problems, and they'd put on a lovely show.
Cheers,
Neale
