2 part answer.........
Mixing different thickness's (or even lengths) is a common practice for "close to bottom" hunting. For example on my suitcase JBL knock-down gun I have 3/4 and 9/16 band and also 2 different lengths. By loading only the 9/16 soft band I can get into a rocked up fish and not destroy my shaft tip with too much power. But I can also load both bands for full power and have enough punch for a mid water fish.
The 2nd part of the answer deals with "consistency". On my every day normal Koah gun my 2 bands are both 9/16 and are very exact lengths that I cut and tie myself. I change these band out every 4 months even though they are not broken or cracked. After diving every week and multiple loadings, the range and most importantly the stone shot accuracy begins to degrade. Bands are cheap, losing a $150 fish on a crappy shot is not. A chef will not serve a filet with a hole in the middle of it, accuracy is everything.
In short, try different types of setups for your gun. If you like the results STICK WITH THEM. If your accuracy is terrible, don't prolong the pain, change your setup immediately. When your gun is dialed in, your confidence will be so high that you will never miss another shot because you know exactly when your gun can't hit it and won't pull the trigger. You spend so much money just to get to the dive spot, don't go cheap on $12 of rubber.
Test > Change > Dial it in.