Thank you in advance for answering my question.

Since I am new to this, I would like to know do I really need have additional amplifier? or the existing setup is ok?

Here is my setup
Home Theater 6.1
AV Receiver - Denon 1908
Front Sides - M60 V2
Central Speaker - VP100 V2
Subwoofer - HSU STF-2
Rear Sorround - Speakercraft AIM7 TWO (3 Ceiling Speakers)
+
Zone 2 - Whole house Audio
3 rooms home audio (total 6 Ceiling speakers)
All are Speakercraft AIM7 TWO ceiling speaker
with Speakercraft VSI60 Vol Ctrl (Impedence Matching)
with Speakercraft SWTS-4500 Speaker Selector

Zone 1 is certainly fine unless you tend to play pretty loud and might want some extra wattage to keep it clean. I'd like to understand how the three rears are wired and amplified though before I say for sure.

Can you describe how zone 2 works a bit more? Is zone 2 set to be a pre-out for the Speakercraft system? In other words, does the speaker selector or volume control provide provide the additional amplification?

I would think it would have to in order to drive up to six speakers at once, in which case you should also be fine as then the Denon isn't doing any of the real amplification work.