Plan on getting a leopard gecko at the end of september (hopefully) and about to get my cage setup and I have a few questions.
1) does the under the tank heater stay on at all times? (nothing in my research said anything about it)
Just make sure it is hooked up to a thermostat

I do not do a night drop for my guys.
2) are those half coconut shells aka "dome homes" hides big enough for an adult leo?
3) I know you use calcium with Vitamin D3 for dusting food but should the little jar of calcium I leave in the cage at all times have it as well?
No, leave pure calcium in the tank.
4) still debating on 10 or 20L terrarium
If you have space for it get the 20L, it's a good size for an adult and all the 'stuff'.
5) I know loose substrate is a no for leos so I can I clutter the cage with natural looking hides,wood and rocks so it's not so plan?
Yup, as long as it's sturdy and there is still good floor space. Keep in mind though, crickets may get in that stuff and hide, don't want to leave them roaming about.
( god I feel like a reptile noob! lol)