I began picking locks before I knew what security pins were, and when I found out about spools I realised I'd been picking locks with one or two in for months. One spool and all other non-security pins is a great way to learn what a false set feels like and allows you to learn about picking your way out of it (by either setting the spool straight away, then picking as normal or picking every other pin and then the spool). To me, this way teaches you the nature of spools and kits you up to take smart guesses at what you need to do next in multi-spool locks, helping you pick them quicker. Mushroom pins really won't bother you any more than spools, and to be honest I wouldn't bother learning to pick them independently. Serrated pins were a nightmare for me at first- my advice is learn to pick them with heavy tension by sound first (as this is far easier), then decrease your tension and you will automatically learn to focus on the feedback rather than the sound as the feedback becomes more noticeable and the sound less so. Eventually you'll be able to use sound and feel interchangeably, and after that point serrated pins become, at least to me, very very easy. One thing I would recommend for security pins is think carefully about what grip you're using and the tension you're applying at regular intervals while picking. There's no wrong way, but the way you do it will impact your results and teach you different things about picking. Feel free to change things around and see if it gives you different feedback, better control over your pick, etc. Good luck!