Your existing toilet flow to the soil pipe already has a T-joint.

To add another T-joint, you need to raise the platform quite high for allowance for the new joint.

By the way, I dont think is easy to add the new joint to the existing soil pipe. The plumber may need to use heat to soften the existing soil pipe to push it into the new joint. Take into consideration that you may find leakage/smell entering your existing toilet if the plumber dont do it properly.

I dont think is neccessary to do any water proofing for existing toilet as the plumber can drill a hole of the size of the new pvc pipe like how cooker hood is installed for air to escape to outside the house.

I cant tell the allowance as it depends on the outlet to the soil pipe.

But it is never adviseable to have t joints sitting next to each other as there will be no room for adjustment if mistakes are made.

I have to caution you that the work of inserting the soil pipe into the new t involve shaking and pulling side ways. During the process, it may cause leaking around the joint of the soil pipe near the ceiling in the long run or even worst, it may dislodged due to poor workmanship of the developer. Things are weird sometimes ... if you dont touch it, nothing happens .. and vice versa.

By the way, are you are doing additional toilet only or to include bath? Was wondering why water proofing?

On the lighter note ... it reminds me of my childhood time when I used the raised type bucket toilet.