If you put a line of adjacent command blocks to be executed at the same tick, they are not activated in the powering order, firstly powered (strongest power) firstly executed, they are instead powered in some situations that seems to be unique, here what I've seen :

• the same system in two different worlds at the same position does not change activation order, should does like now.
• the same system made at the same position, with block at the same position but with blocks placed in a different order does not change the activation order, should does like now.
• the same system in two different position (in the same chunk) change the activation order, should not.
• the same system at the same position depending to chunks but in different chunks change the activation order, should not.
• the same system with a different rotation change the activation order, should not.
• the same command blocks at the same position but with redstone source at a different position change the activation order.

If we additionnaly put command blocks around the directly powered block, they should be activated depending to their relation with the directly powered blocks (always top > north > south > east > west > bottom for example)