I wouldn't mix old and new fluid...also, transmission fluid isn't hugely expensive, so it would be worth your money (if you don't have the money for a new tranny) to just replace all of the fluid in the transmission. Check out this tutorial for some pretty easy steps to help you change the tranny fluid.

One other thing to check would be to see if your clutch master cylinder is leaking fluid anywhere, cause that could cause those kinds of issues...