Endlers grow to just over an inch, and they don't take that long to get there. Fish in shops are youngsters and are always smaller than the adult size.
Be careful with 'bottom dwellers'. Your tank is a cube and therefore has a small bottom to volume ratio. Bottom dwellers need a bigger footprint than a cube can offer. Endlers will feed off the bottom - I have 2 remaining from a group (which I have just realised I have missed out of my signature) and they do - so food that ends up on the tank bottom will be eaten. If you have an algae problem, nerite snails are the best solution if you don't mind them. Nerites don't look at all like slugs with shells

.One of the larger species like zebra nerites or a couple of the smaller black and yellow striped ones.
On the subject of food, fish need a lot less than most people think. As a very rough guide, they should be fed the amount of food equal to one eye per fish. A fish's stomach is about the same size as its eye, so half a dozen fish need feeding the same amount of food as half a dozen eyes.
Be very careful about advice from shops. Big chain pet stores tend to train their staff in how to sell things not how to care for the stock. Too many of them tell their staff to make answers up rather than lose a sale. You do get some knowledgeable staff but they are few and far between. The fish-only shops can be as bad, they tend to push chemicals at you to solve a problem rather than sort out the root cause. And I have not yet come across a shop that believes in fishless cycling (though one shop owner did ask me about it!)
When you tried to attach the heater, did the tank have water in? Suckers don't usually work on dry glass.