Those plans are 2x8's which have a maximum span of 11' 10" if SYP. The plans say the joists are 141" which is 11' 6", so there is no span problem. A 2x8 has less deflection than a 2x6. You can't change to a 2x6 without changing the design.

One thing I do not like about those plans is that there is no positive connection to the ground. By this, I mean I could stand on the side of your deck and lift it up (if I were strong enough). That's because the blocks are not attached to the ground, and the deck is not attached to the blocks. Also, if you're in an area with frost heave, the soil moves with changes in temperature. The deck blocks will move up and down, and eventually (maybe in the first few years, depending on how well you prepare the site), you will have uneven and wompy deck footings, leading to a deck that is not evenly supported.

You can solve both of these problems if instead you use 2x8 lumber and pour footings that support the perimeter of the deck. You need a footing at each corner to support the short side. And then since the outside joists are too long at 16-18' each, you need a support at the midpoint of each of these. The footing has a metal fastener in it when you pour it (or attached after with bolts when it has cured), and then the deck is bolted to the fastener.

You also need to calculate the load to make sure you have enough footings. If the deck is 18'x10', that is 180 sqft. The rule of thumb is 40 pounds of live load (a person walking or an object on the deck) per square foot plus 15 pounds of dead load (weight of the deck materials) for a total of 55 lbs per sqft. If you have a footing at each corner and one at the midpoint, this creates six sections of 30 sqft each, and each footing must support 55*30 = 1650 pounds. Use this footing chart to determine what size footing to pour based on your soil type:https://www.decks.com/how-to/articles/deck-footing-size-chart

I would add 2x8 joist blocks (short pieces) between the joists as well:https://www.decks.com/how-to/articles/deck-blocking-and-bridging

All lumber should be rated for "ground contact" treated wood, because you are so close to the wet soil that it is subject to rot due to poor air circulation. "Ground contact" has additional protection compared to normal treated lumber.