When it comes to the STP active timeline most people associate the beginnings of STP/MJY with when Scott and Robert started writing together in 1986. While it wasn't exactly STP, it was setting the foundation for it. I think Eric joined them in 87/88, Dean mentioned in 2008 that Scott, Robert, and Eric had been playing together for over 20 years. I think Corey Hickock was the guitarist around this period, I'm not sure. Dean then joined in late 89 or 90 and that's when they started going by MJY. Hopefully Scott's book clears some of this up, because I've read everything there is to read about STP and that's the best I can come up with.
Also the former drummer only played one song with them at a show, he didn't play a full show.