The OPTIMUM rpm for all 175's is 5500-5600 unless it's a 2007, in which case it's 5300-5500. If motor is newer than 2007, your rpms are a bit low for that light load you're running. Check out this web site for a ton of good info on your motor:
http://www.etecownersgroup.com/post?id=5396435
Step 1 before changing props is to make sure the tach is accurate, but if you're running the I-Command tach which gets speed from the ECM, you're good on that. Then make sure the motor height is right (AV plate should be out of the water when you're up on plane). That typically equates to the plate being about 1-1.5" above keel for a transom-mounted motor. Then you'll have to decide what performance characteristics are most important, i.e., do you only want max top end speed and high cruising speed with a light load, or do you care about good low speed performance with a heavy load in rough seas, like being to hang on plane at low speed, getting on plane easily with a heavy load (good hole shot), and good cruise fuel economy? I'm guessing you want the latter, or else you'd be running a 300 on that boat! All props are compromises; no single prop will do all those things equally well.
The Rebel is a great 3B prop with lots of blade area and stern lift that will help low speed planing and cruise fuel economy, but most of the guys on the owners forum running that prop on the small block 2.6L V-6 are running bass boats that are much lighter than your rig. I'm giving up some top speed and fuel economy with the 4B 15x15 PowerTech prop I'm running, although it has an awesome hole shot, will plane at about 12 mph, and will turn 5400-5500 pretty much regardless of load. My boat is quite stern heavy with the 30" setback bracket, so I need a lot of stern lift that the 4B provides. I asked my dealer about the Rebel, thinking it might still have enough stern lift but would burn less fuel with the lower drag of only 3 blades. He said that his experience was that the small block V-6 didn't have enough torque to turn the Rebel on a boat like mine, and that most guys running it on large boats had the big block 3.3L V-6.
As I understand the late model 21, it's more of a shortened 23 with much higher deadrise than a lengthened 20, so that's a pretty heavy boat for a 175. You might be better off with a BRP 4B Rogue or Cyclone, or a 4B PowerTech. The latter has a pretty good web site with a prop selector where you enter existing boat, motor, prop and performance info, and it'll recommend a prop. It's still not perfect though, because they recommended a 15x17p 4B prop that was ~200 rpm too low in max rpm, so they exchanged it for the 15x15 I have now. It's VERY important to do all testing with the type of load you typically plan to carry. I always tested with a full gas tank, and about 50 gallons of sea water for ballast in the live well and in a couple of 70 qt coolers.