I never have issues using 2 lengths on rear wheels, because I also use 2 different gauges, so need different spokes anyway.

On a typical pair of wheels build I typically use the same (lighter) spokes on the left side rear and the front, and the shorter, stouter spoke on the right rear.

Your question about a strength gain using lighter 2.0/1.5 spokes is tricky, and turns on the definition of strength.

If you're looking to reduce elbow breakage, then yes. But it will be very hard to find tensions on a dished rear wheel such that the right isn't too high and the left too low at the same time. Also, the lighter spoke will allow more lateral flex, and that may offset any advantage.

If you want to go lighter, while producing a 1st class wheelset, use the 1.5s on the front left rear and the 1.8s on the right rear. The reduction in gauge on the left will allow decent elongation at lower tension. By lowering the left's minimum tension, you won't be pressed to bring the right as high.

BTW - the key to spoke length forgiveness is the nipple. Since you want at least 1-2mm thread engagement in the head of the nipple, that means that the shortest spoke you can use will end about 2mm short of the top. If the nipple doesn't allow overrun above the top, then you have a narrow 2mm margin of error in spoke length. If your spokes are long on one side and short on the other, then that 2mm target will be too small for both to land within it.

I carefully seek out nipples which allow plenty of overrun, preferring 3mm. When I'm building hollow rims the overrun provides a 5mm target window (-2mm - +3mm). I don't take advantage by using the same length when I want 2, but it does allow me to aim high, so my finished wheels have full thread engagement into the spoke head.