Being in a salaried job while working a side hustle (small business/freelancing/passive income), is it legally possible? At what point do you have to (1) Report Income (2) Register your small biz?

I dont think legally your "main" employer can do anything as long as your side hustle doesn't interfere with your main job, or conflicts with the company's interests. Obviously you have to make sure that during the official work hours, you are not working for your side hustle, that you do not steal any information etc etc.
For all income derived in Singapore, you have to report them for tax purposes. Even gifts received by influencers are considered "income".

I believe the government just wants to tax your additional income somehow. So if you're declaring personal income tax, you are generally fine (although in some instances, corporate tax is lower than personal income tax, and having a business entity shields you somewhat from liability). Register your small biz once you feel that you want to get serious about it.

I believe you have to check with your HR on moonlighting if you want to do it the legal way. Because different companies have different policies on this. Some are lenient while others will even spot-check.