I don't see any problem with the belt or pouches. As mentioned above, depending on your color scheme, you may want to use some leather/shoe polish to darken them up a bit, but I think that would be okay to use right off the shelf.
A lot of us use the leather Mauser pouches that are widely available through military surplus websites, and if you look back at the Original Trilogy (especially Episode 4 when the budgets were low), the screen-used props were cannibalized World War II-era gear that was slightly modified to look "SWarzy", but in some cases was off-the-shelf equipment that was roughed up to look used.
As to whether you could use that in lieu of a girth belt or sash, I'd like to see a mock-up of your kit before making any comment. However, if you look at the CRLs, a girth belt is not required, although a lot of people wear them. If you have a one piece flightsuit, you may not need one and the ammo belt would be sufficient.
From the look of the picture, it appears that the pouches probably cover 3/4 of the way around your body and the belt buckles in the back. I don't think that is a problem, and if you wanted, you could easily cover up the buckle with a kama or cape.