My brother-in-law made all of my pegs for me by ripping them on a table saw - into octagonal shapes. From the 1-1/8" boards I gave him, he could make over 500 16" long pegs in a day (easy). Flat-to-flat was less than an inch, and corner to corner was slightly more than an inch. (As Mark Anthony suggests above). As long as the pegs meet this basic criteria, your tolerances (dia. of the pegs) can be a little sloppy. Not all were perfect octagons either. With that said, some drove harder than others (hardwood into hardwood). We sorted the pegs by size and took more than a few of his bigger pegs to the belt sander (the ones that were nearly an inch, flat to flat). Black locust pegs were, by far, much better than white oak pegs. (rarely split when driving). Walnut pegs weren't worth a crap. No doubt, these pegs fit very tightly - if you drove one completely through the beam, the cross section of the peg emerging (extruding!?) from the other side of the beam would be more like a circle than an octagon.

I have no experience driving them into softwood, but I imagine they would work great. Ours were sometimes a pain-in-the-butt because they were hard to drive into the hardwood. Rounding both ends of the pegs helps. The leading end of the peg should be rounded for obvious reasons (or tapered for drawboring), but one of the guys helping me (who had never worked on a timber frame in his life) discovered that it also helps to round the end that you're hitting with the persuader - the pegs (ours hard dried) tend to split less frequently (but it's rough on your persuader!).