That is correct there are many combinations but the 1e step is to find combinations with high correlation, like T-Bonds
against 10Y Tnotes etc.
Then the contract value must be near equil, or for example, 1 DAX against 3 Euro Stoxx 50 etc.
CSI data users can view correlation daily.
Further, it is our experience that spreads based on weekly closing prices are doing better then daily or EOD action, because less trades, less slippage, so you new enough volume too.
To understand something about statistical arbitrage you for the tour on
www.2hedge.com
I know Barcharts.com has US futures in a limited way and
www.tradetrack.com and
www.fxtrek.com doing something with US stocks and FX.
QQQ, SPY etc. are also nice, but their margin is higher.
Hope this answer your questions.
Regards,
Gann