Can the software be used for markets other than Forex?

Yes, it is possible to use the NNFX Algo Tester in other markets, although in some instruments the lots calculation may not be as accurate.

In such cases you will get a notification "WARNING (803) -> The QUOTE/MARGIN currency SYMBOL will be used as account currency for the trading volume calculations."
In other words, the calculation of lots will not be as accurate as was supposed and will be an approximation. The tester includes a USE FIXED ORDER SIZE option if you wish to remove this warning.