My comment is: I hope you have a bl**dy big tank!
Guppies and neon tetras aren't the best of mixes - the tetras may nibble the guppies tails (look like lures to them and they are insect eaters, of course). Also, guppies like hard, alkaline water with a touch of salt in it, and tetras like soft, acid water and don't like salt. But I've known people who've made it work. I actually find black neon tetras (no relation, despite the name) are a little less nippy.
As for the bigger fish you want to add please check on their requirements and adult size. Discus are certainly not beginner fish - they cost a fortune and ideally require RO water to be kept successfully. Bala sharks are very large schooling fish, totally unsuitable to most home aquaria. Chinese Algae Eaters are large, vicious, territorial fish best kept in a species tank on their own or maybe with other territorial fish who can cope. Eels I know little about except they are generally big and predatory.
But before you do anything at all, even with neons or guppies, please make sure you've understood about how to cycle a tank. More fish die through ammonia and nitrite poisoning (which occurs in an un-cycled tank) than any other cause. Besides, if you want discuss or eels one day, you're going to have to understand how the ecology of a tank works in depth. Please follow the link in my sig for further information.
