Why would you want to use only the cross slide, if in fact you have a compound rest; the result of using only the cross slide is that the tool cuts on both sides using this plunge cut method; what happens is that the two chip streams converge at the point, interfering with each other and causing galling in the cut and a very unpleasant looking finish on the flanks of the thread. An advantage to using the compound for feeding is that you zero the cross slide on every cut and do your successive cuts by feeding the compound in, with no numbers to memorize for each new cut. Some folks like to compute the distance the compound is fed in to size the thread, or use the figures on the "fish tail" thread gage; this may work in an perfect universe and with sharp vee threads, but if you use a flat on your threading tools, it does not. Best is a thread mike, and the tables published in machinist's handbooks, but for me, a thread ring gage or a nut that has had a tap run through it is all that is necessary to insure a good thread fit for most purposes.
Lastly, I think, no, 20tpi is not fine enough to plunge in straight, and personally, I would not recommend it for any threading.