Mites occur out of the water mostly, as far as I've noticed. I see them on my water surface on an infrequent basis. I've found mites in microwaved peat, on leaves, just about anything that isn't fully submersed.

The ones you got leefhker are indeed springtails, in particular the globular springtails, family Sminthuridae. Not sure if we have any in Singapore because some are introduced pest species.

Gammarus aka scuds, are good for fish tanks since they are scavengers, but in shrimp tanks as Navanod has pointed out, they compete directly with the shrimplets for food. I don't know how to eradicate these other than manual removal with a siphon tube, because any medications used to kill them will affect the shrimps too, given their similarities. You can consider using a copper based medication but this will kill your shrimps along with the Gammarus.

Siphon them out, or trap them and keep in a separate tank. You never know when they'll come in handy as fish food.