Not necessarily the right time now, DYOR. If you want to take a gamble, then sure, feel free to add juice to the next downleg. Rate of shares are low? I assume you mean they're cheap. Cheap is relative, would it still be cheap 20% lower? How about 50% lower from now?
Purchasing from different sectors won't give you much protection when bloody EVERYTHING is falling. With $5~10k you can't afford to diversify. Buy in 4 sectors? $50 round trip means you're down $200+slippage. Gotta make 2~4% just to break even.
Read the beginners section; theres a lot of good stuff there that may hopefully answer your questions.