TBs are pretty hardy fish, and if you bought juveniles in your current tank setup then they'll be okay for a little but when they start growing they'll go nuts or die of stress in such a small tank. Just to let you know I made the same error but intentionally cycled my tank with 5 juvenile tiger barbs. None of them lived and here's my conclusions:
Juveniles are weak and easily prone to death by stress, condition changes, uncycled tanks.
Tiger barbs are shoaling fish. Many people say 5-8 is okay, HOWEVER based on what others told me who have kept barbs long term, these guys THRIVE in groups of 12+++ The more the better, and when you get in really high numbers they form their own seperate gangs and get really exciting to watch.
They are mid-dwellers and love to swim around, so leave the front end of your tank open and having a long tank is better than a tall tank. So put all your hiding places/plants in the back, the weaker ones will need places to hide but the open swimming space is very important for their activeness. I recommend AT LEAST a 20 Gallon tank. I have one now and it sucks I'm in need of an upgrade because I'm already at capacity and I can see my group is fun but adding more barbs to their shoal will increase activity/enjoyment.
They are semi-aggressive fish. The more in your shoal the less they bother other species. They go well with sharks (balas, rainbows, redtails) however do not mix redtails and rainbows (research those or I will post later on). You can mix other types of barbs with them (cherrys, tinfoils, etc) Some gouramis will work too. Danios are also fine. Bottom feeders and top-dwellers will work too. Just stay away from fish with very long fins (fancy
goldfish, bettas) they will get raped by the barbs.
FEEDING - you can feed them shrimp pellets, flakes, bloodworms. I mix it up between the 3 and when they get bigger ill eventually do vegetation too.
You mentioned planning on upgrading as they grow. IMO it is best to just by a large tank from the start.
1. you waste money on constantly upgrading tank+supplies
2. it's a pain to start up new tanks over and over
3. You will be happy if you get it right the first time.
my 20G was a good start but I soon learned and wish I started with a 40-50G tank for the community I want to build.
Overall, do not be discouraged if your barbs you have now die. I've purchased 11 Tigerbarbs over all and now I have 5 which made it through the cycle and are doing great. Good luck with your new hobby and keep us posted!