The answer you're looking for is very simple. There are two different size Nitro cams; Nitro 2.0 and Nitro 1.5. The Nitro 1.5 is for shorter draw lengths, 25" to 28". The Nitro 2.0 should cover the upper range, 27" to 30". Notice there is an overlap between the two so if you fall into that range you can choose either cam.
As for using a draw stop? Yes, the draw stop can be used as Wabbit explained, but it is recommended not to shoot the bow with the draw stop set too long. When the cables hit the flat spot on the modules at full draw you should not set the draw stop much longer. There have been cases where the draw stop was not in place .and the bow drawn past the flat on the module. In some cases the bows have locked up at full draw which can be very scary and dangerous. In such cases the bow must be put in a bow press to relieve the tension on the cables.
If You're Not Living on the Edge You're Taking Up Too Much Space
Martin/Rytera Staff Shooter
PSAA Life member, UBP Life member
PADI AOW Diver