Get the biggest tank you can afford. It will be much better than the tank they are in now. I'd upgrade as soon as you can. Angels grow pretty fast. My first angel was about 4 inches tall and about 3 inches long when I first got her 8 or 9 months ago and now she is 7 inches tall and about 6 inches long....huge! They grow pretty fast, especially when they are small. If you leave them in the 5.5g tank too long it will stunt them and they could die an early death.
Glass cats are schooling, so you'll need more than a couple more to make them happy. A group of 6 or more is best. Get a 30g if thats the biggest you can afford....if you can manage a 40g, thats even better. Wait until the angels form a pair, keep that pair and return the other two. A 30g isn't a good forever home for 4 angels....the most I'd keep in there is 2.
Neons are the angels prey in the wild, so if the angels are good size and you add small neons, they may end up as snacks. Most people who keep angels and neons together successfully, put the neons in the tank first.
Get the new tank and then worry about what to add to it. Oh and don't listen to the advice from the LFS