zoekin8yg is right about the cories, while they "can" live alone, they are much happier in groups and they do shoal.
also, many reports have been writen that bettas andg guppies dont get along. the females would be better, but theres still a risk, again, it can be either or with the platies, some bettas wont bother the platies, others will.
the snails do produce a lot of waste, and good filtering is needed, which creates water movement which bettas dont like.
30L is only about 7g, not much room, i belive platies and guppies need atleast 10g.in such a small amount of room, there is a high risk of aggression from the betta.
i would go with either 1 betta, a few pygmy cories and maybe 1 apple snail,
OR if live bearers are what you want, a few endlers and cories, and an apple snail.
all that you have listed is over stocking the tank.
personally i would just have a betta, a few cories and an apple snail, maybe a few shrimps.