Howdy.
A 20 gallon long is fine for a mated pair of any of the small to mid-sized Neolamprologus species, such as your brichardi, buescheri, or caudopunk, or for a mated pair of Julidochromis. These are substrate spawners that form solid breeding pairs, unlike Lake Malawi mbuna which need harems for successful breeding. It will also work for a small group of 6 or 8 shell-dwelling cichlids, such as multies. Larger Neolamprologus species, such as N. tretocephalus, N. sexfasciatus and N. tetracanthus (which all can exceed 6") need a 40 breeder or larger for a pair. I'd also recommend a 40 breeder for a pair of calvus. One thing to remember if ya want to breed the N. brichardi, they form community breeding groups. As fry mature, the older juvies stick around and help the parents raise the next batch of young. My last breeding pair of brichardi and their offspring had an entire 55 gallon tank to themselves.
A 20 L is too small for an extended community of Tangs. Ya really need to invest in a 55 gallon tank or larger. The calvus can potentially reach 6", and 4 1/2" to 5" is pretty much a given. Not sure which species yer referring to as a highfin synodontis, but I suspect its either S. euptera or S. alberti . Both can reach 8" and will become too large for that tank, as well.
I watched the video, but there's a coupla issues with the request to determine sexes. First, the fish ya currently have look too small to sex, even by venting. Second, the species you have do not show extreme sexual dimorphism. There is little visible difference between the sexes, if any. In order to successfully determine the sex, you'll need to vent the fish (turn 'em over and look at their nether regions).
WYite
