No, the Serpa CQC won't work inside the waistband.
The paddle is for the convienence of being able to take the gun & holster off without having to unbuckle/unlace your belt. Most paddle holsters ride a little farther off the body, and are only well concealed under jacket/outerwear garments.
A standard belt holster ride a bit tighter to the body, and is obviously more secure than a paddle.
Inside Waist Band is another design altogether, and I would personally recommend a leather holster for IWB carry. The Galco Royal Guard is a good example of a popular style IWB holster.
The BLACKHAWK shoulder harness is designed to be used with a Serpa CQC holster that you already own. You would remove the paddle or loop back and attack the harness for shoulder holster carry. I can't speak to the quality, but I have been pleased with other BLACKHAWK gear. In other shoulder holsters I can +1 the rigs from Galco Gunleather.
I can comfortably recommend leather and polymer holsters, and think the CQC would serve you well, especially in the belt slide format. I also like Galco Leather holsters.
I can also +1 MattG about passing on the ninja gear. A quality strong side holster is the gold standard for law enforcement for a reason, it works. Also, invest a little money in your carry gear. People spend hundreds (even thousands) of dollars on a piece, and then buy a $12 nylon holster with a cheap plastic belt clip. I'm not against frugal if it works, but know that often quality has a little extra cost.
Galco Combat Master:
*I'm surprised pic is hammer down carry...only one on Galco site I found like that.
Galco Royal Guard: