Okay, I've not answered one of your queries before, I'll bite.
Having complementary Primary and Secondaries is helpful, but technically that could be any combo. You need to understand the Secondary Powerset a bit, as it's often the one that skips Powers. Also helps to understand Control Powersets in general too, as Powers should be skipped there too. For example, except in rare cases, I never take the AoE Hold as it has much lower Base Accuracy and a very long Base Recharge, making it a pain.
Just pick a combo that appeals to you. Make a build. Look it up in the Controller Forum to see what others have said. Ask about it.
Here's some combos I know work, either personally or from hearing it from many reliable sources:
Dark/Dark
Fire/Kin
Fire/Rad
Ice/Cold
Illusion/Dark
Illusion/Rad
Illusion/Storm
Pick the one that appeals to you. Make a build, ask about it as I mentioned above. Make and play the Toon. Judge the Toon on whether you enjoy playing it.