I'm the last person to ask about frames, but I have a basic understanding. Say if you see something that's -10, you can be hit for free with any 10f move (ie, jabs, and sometimes in yoshis case flash). The more negative, the more punishable you become, and eventually the opponent starts getting free launchers. That's where the risk/ reward factor comes in. Is it worth throwing out this move? Will it kill the opponent? If it's blocked, will it get me killed? Gotta ask yourself those questions.
Yep, SS1 is good. It's safe I believe, and launches on counter hit. Also, if you use it after b! you get a short wall carry or you put the opponent face down feet toward... not the best position to be in.
Word of advice, since you're learning the basics, go through yoshis command list and get yourself familliar with his regular stance, don't use anything else unless it's for combo purposes (ie b! b2,1 kin f+2 that you've been working on). Once you feel your basics are stronger, then start moving onto kin, med, ind, dgf, flea, and NSS. Don't try and tackle it all at once... yoshi is a very difficult character to learn, and at times you'll find yourself being confused by your own moves. I found it best to learn one stance at a time, despite the fact it seemingly takes longer, the information soaks in better and you will progress quicker.