I will assume your 10g is the basic 10g that is widely available in NA, so it is 20" long. I know it may seem larger than it actually is...I remember my first tank, a 10g, decades ago that seemed "large" too, then I got a 25g and I thought I'd never have enough fish in it. We soon learn that fish grow, and need space, but also the water quality is impacted by fish and space limitations.
I whole-heartedly agree with previous members that a 10g is not sufficient space for platy, and any pleco including bristlenose. With your hard water, Endlers (stay with males, females will mean fry everywhere in no time) or guppies (males here too) are good choices. Some tetra will manage in harder water; the Pristella Tetra comes to mind. But this is only if no endlers or guppies, not both.
There are considerably more "nano" type fish options with soft water, but small sized fish have more delicate physiologies, and often will be wild caught, which makes attention to parameters very important if they are to be healthy and live normal lifespans.
Byron.