I should say, I'm no expert...By ANY means. But;
No flasher. Use a dummy flasher off your down rigger ball if you want a flasher but you don't want a flasher in front of your plugs. They have to swim.
If you're running one rod, then run it back however long you want. Some people go really really long. I tend to at least double the distance from the clip compared to when I'm fishing a flasher: So if I run 15-20ft with a flasher, I'll run 40+ feet with my plug. It gives it more freedom to swim around. If I'm running two rods with different offerings I'll shorten it up a little. I'll run the flasher gear at 15ft or so off the clip and the plug 30ft or so. And make sure you're not running the plug rigger deeper than your flasher rigger. Since the plug floats, you can get yourself into one beautiful mess after making a turn. Make sure you clear your plug line if your flasher gear gets hit too. You don't want a buoyant lure dangling behind your boat when you're hooked up.
Speed: I tend to troll the same speed as I do when I'm fishing flasher gear. But a lot of guys like to fish plugs fast! I probably go no less than 2.5-3 knots when I'm running plugs. I like to have a 45+ degree angle on my down rigger line for everything I use.