Hello,
I am from Africa and have a source of free electricity.
What hardware is recommended for me? A used S7 will do? How much can S7 profit under present condition?
Also can Antminer can mine other curenncy?
To answer your 3 questions:
1) be carefull, there is no such thing as free electricity... Somebody is footing the bill, it might not be you, but in the end, somebody is paying for it.
2) if you have "free" electricity, eventually most ASIC's will ROI... Unless they break first. For an S7, you can see here:
http://www.coinwarz.com/calculators/bitcoin-mining-calculator/?h=4860&p=1210&pc=0&pf=1&d=559970892890.83800000&r=12.50000000&er=1891.11260000&hc=0.00 for the average income at current diff, block reward and BTC price
3) An ASIC can only be used to mine one algorithm. Bitmain makes two kinds of asics, most of them mine sha256d coins only, but they have started producing a product line to hash scrypt only...