I've just bought a 45 liter 50x30x30 aquarium. I was wondering would 2-3 colisa lalia (Dwarf gourami), 1-2 ancistrus and 6 ember tetras (Hyphessobrycon amandae) fit and be happy?
It's possible to keep those fish in that tank, although I would go with only 1 bristlenose pleco since they can get 6 inches long.
The worry I have with a slightly overstocked tank like this is that there isn't much room for error if the water parameters fluctuate. If there is waste buildup from the fish, or leftover food, it will decompose into the water, causing an ammonia spike that could be just enough to stress your fish and kill them.
To keep that from happening you're going to want to invest a good filter, perhaps one that's slightly overpowered, and perform more frequent water changes to keep the water parameters stable.
If you want to make the tank require less maintenance, I would suggest not getting the pleco. The tank is small enough that a snail should be able to take care of algae if that's your concern. It also won't hurt to use an algae brush to clean the algae off the glass yourself. Since bristlenose plecos get to be about 6 inches, it will be the largest fish in the tank, and plecos in general produce quite a bit of waste that you're probably better of not dealing with in that size of tank.
The tetras would be fine. But I really think for the others you'll want at least twice that tank size. My concern is that the load from 3 gourami and 2 bristlenose plecos would be too much for a 45l tank when they start to reach adult size. Plecos in particular generate a lot of waste.
I would definitely not add them in all at once. That will overload the tank immediately.