Note that just leaving the tank for a few weeks won't do anything. You either need to add one or two starter fish or (better) choose the path of fishless cycling- adding liquid ammonia. This is by far the most humane way of preparing the tank. It may take a few weeks before you can get your fish in, but after all- you would be prepared to take time to set up and get conditions right for any other pet, wouldn't you, so why not for fish? Since you've already got a tank, you should be able to speed up the process by squeezing your old filter over the new one.
I am just going through the fishless cycle myself, and though it gets a bit boring weaiting for the nitrites to go down, at least I've got plenty of time researching my new fish.
Which brings me to question no 2- fish list. Of the list you mentioned:
electric yellows are African late cichlids- they can only be kept with other African cichlids from the same lake, and you need to research any tank mates to make sure they are compatible. This is partly because they have very specific requirements about things like water hardness, partly because they can be very aggressive if kept with the wrong companions; cichlids from the same lake have some chance of understanding each other's language and keeping out of the way of trouble- fish that don't understand might get ripped apart
Basically, in fishkeeping, you can either set up a Malawi tank, a Tanganyika tank, or a general community tank which does not contain African lake cichlids (but most common South American and Asican fish can be mixed- if they are otherwise compatible)- but mixing them all together can have disastrous results.
African cichlids need a tank with hard water, stone rather than lots of plants, plenty of hiding places and very good filtration- 125 ltrs/33 US gallons is probably too small unless you go for shelldwellers.
Angels are South American cichlids which will go fine in a community tank as long as it is at least 18 inches high and they are not kept with fish small enough to fit in their mouths- neons can be at risk here. Most angels are small when you buy them- but there aren't any that stay small.
Neons are beautiful fish, but I wouldn't add them until the tank has been up and running for at least 3-6 months without problems as they can be sensitive.
Tiger barbs are lovely fish. They need to be kept in groups of at least 6-8 to spread aggression as they are very lively and work hard at maintaining group hierarchy. You wouldn't want to keep them with anything with long flowing fins (like guppies), and angels might also be at risk of fin-nipping. But they should hold their own against your RTBS. The only thing that worries me is that a 125 ltr tank might not be enough room either for the RTBS or for a whole school of tiger barbs (don't get fewer than 6 barbs)- I would ask about this in the cyprinids forum.
Community tanks often seem to fall naturally into two categories: the peaceful and the rambunctious. A peaceful tank would be say, guppies, the gentler tetras, angels (though they can be quite aggressive) or gouramis gliding peacefully through the water, maybe some corydoras at the bottom. A rambunctious tank might contain a large school of barbs and an RTBS. I have had to regretfully stamp on my love for giant danios, as I realise they would worry the life out of the pearl gouramis I am planning for my new tank. In your case, since you've already got the shark, you might want to think about fairly sturdy companions.
