I make and use regular fishing line for leaders and tippet all the time. I make twisted leaders with small barrel swivels on the end, and then I tie on a lighter pound test for the tippet. For example, my 6wt leaders usually are twisted Cajun Red 12lb test, then I tie on either 10lb or less as a tippet. It all depends on the targeted species and rod weight.
Bottom line, yes, regular fishing line works just fine. I don't think I've actually ever used or even bought "fly fishing" leaders and tippet. Except furled leaders. Bass, gills, and crappie aren't too picky about leaders. I've even ran straight pieces of 10-12lb Stren as leaders before. Never any issues.