-Is there a specific or desirable characteristic that a steel single pivot should have
I recon somewhere between 110% and 130% anti squat. Depends on your riding style, for me 110% is fine, my wife needs more because she is a violent pedaler hahaha. with a single pivot try to get as much progression as you can, it probably wont be much but you don’t really need that much. if you get somewhere between 5-10% it should help.
-Is building it without a linkage software a good idea
Not really, linkage x3 cost $25usd. How many hours are you going to pour into this bike build? Best to get the software so you have an idea of what you are going to get at the end. It also means you will be able to use your bike as a reference point for making the next bike even better.
-What are the most important guidelines I should go with
Dunno about most important, but ballpark leverage ratio should be about 2.8:1
Choose a travel range and pick the metric shock size that fits that travel range.
-Do you design one with a specific shock in mind? air vs spring
Air sucks, use a coil (opinion)
-Is there something, you should definitely avoid during designing a frame like this
I use a 216x63 shock because I had one lying around on my first frame. That was a mistake, its outdated (hard to find modern replacements cheap) and in my bike it’s overdriven with about a 3.2:1 leverage. Its not the end of the world but it means I needed to custom tune the shim stack to get it working nicely which is a hassle.
Here’s a thread with how some people are doing pivot bearing arrangements:
This youtube channel has some good info too, but I wouldn’t take his opinions on what things should or shouldn’t be as gospel. He does give very good explanations of antisquat/antirise/pedal kick etc: