Don't trust shops. Too many of them will say anything to make a sale, and a lot know nothing about keeping fish.
Your tank is a bit on the small side for lemon tetras. It isn't so much that they'll grow to big to fit as they need more swimming room. If you had a rectangular tank rather than a cube, the length for your volume would be something like 60cm, which would be big enough. For a tank with a footprint of just 40 x 40cm, you need to look at very small fish.
If you really do want to keep the tetras, you could add something like half a dozen endlers - males only as females will give birth to fry every months which would soon overpopulate your tank. With your pH of 8, they would be quite happy.
But before you add any more fish, you need to wait until you are sure your tank is cycled - that is, the filter has grown its two colonies of bacteria. You say your ammonia is zero - do you have your own test kit or are you getting the test done at the shop? If it's the shop, take that reading with a pinch of salt. You also need to know your nitrite reading as that only appears once your filter has grown some ammonia eating bacteria and is also toxic to fish.
8 days is not long enough to have cycled the filter. Even if you added a bacterial starter that actually worked and ammonia solution, it would have taken around 2 weeks. If you don't have your own test kit, you really need to buy one. If you don't want to get a whole kit, get just ammonia and nitrite liquid reagent testers. Then test the water every day and do a water change whenever you see a reading above zero for either of them.
Once both tests show zero, you can get some more fish, but only a couple at a time. There will only be enough bacteria for the lemon tetras and getting too many more at one go will be too much for the bacteria.
Once you have all the fish you want, you can get shrimps. Red cherry would be the one to go for. But only when the tank has been running a couple of months as shrimps are more sensitive to ammonia and nitrite than fish.
As for eating the bubbles, it shouldn't harm them. Fish are not the brightest of creatures, and when they see anything moving in the water they think it's food