I would recommend Rummynose Tetras, they're really cute and a stunner species of Tetras, they also shoal better than the average Neon Tetra, so maybe about fifteen of them would work. As for the Kuhli Loaches maybe six of them, I would up the numbers but I think that's a bit overdoing it, 15 Rummynose Tetras and 6 Kuhli Loaches is probably pushing it since you can't upgrade your filter. I would suggest what Olympia said to do, by just cutting the aquarium sponge in half, removing the activated carbon and just shove as much biological filtration you can in there.
I wouldn't do a Bristlenose Pleco, they poop lots and I don't like them in general. But before you go buying your fish we still need to know what your water hardness is, your pH is 7 which will be able to house both Kuhli Loaches and Tetras, but they both need soft water. So find out before buying fish!