A lot of them are schooling fish and need bigger schools. I'm not sure about the overstocking so I'll leave that up to someone else.
The fish that need to be in bigger schools are:
Neon tetra: 6+
Dwarf Gourami: Although not schooling fish, if one is female and one male then you should try to get a another female, if they are both male then there may be some fighting but they also might be fine.
Harlequins: 6+
Gold Danio's: 6+ (I think?)
Zebra Danio's: Happier in subtropical temperatures but can be in tropical. Need 6+
Rainbow Shark: From brief research, they often become aggressive as they get older and might get to around 15 cm so just watch it and have a backup plan if it does get aggressive.
Cherry Shrimp: Fine
Panda Cory's: Need at least 4 but 6+ is better.
Guppies: Fine as long as they are all the same sex, otherwise you will end up with lots more.
Rummy Nose: 6+
Penguin: 6+
Tiger Pleco: Not sure on them sorry.
If I've made a mistake then feel free to correct me.