2 loose lines of solar collectors crossing from corner to center of the map (making a triangle)to connect to your front line mex is recommanded, as it will expose bypassing cloaked units. And isn't cost much more than the Energy Pylon.
By that mean you can extend the line of solar from there with a few Lotus behind them in case to cover your side's retreating army.
Lotus has decent HP/cost ratio.
I just started a thread about them.
Picket is more useful on cliff where enemy cannot fight back normally.
And you will also need some well placed Razors and Faradays, and 1 Thresher and 1 Chainsaw in the center of a map to limit Licko and casually
You will love merlin, any team shall have one almost any games. It can be used hold fire, you need it to start a combat - break shield and charge, or end a combat - bombard retreating enemy army or force them retreat.
Never use merlin alone consider it is a static