I think I say
●せや(or そうや) for そうよ/そうだ/Yes./You're right./Oh,(I just remember)... etc.
●せやねん(or そうやねん) for そうなのよ/そうなんだよ/Exactly./Yes,(actually you're right)...
●せやな(or そうやな) for そうね/そうだな/You're right./You may be right./Let me see...etc.
I also say
●せやで(or そうやで) for そうなのよ/そうなんだよ(Compared with せやねん, I think せやねん is more subjective and せやで sounds a bit more objective)
●せやて/せやって(or そうやて・そうやって) for そうだって/そうだってば/Right,(trust me, that's the truth...)/(sounds like you're trying to persuade or repeating what you've just said)
●せやかて(or そやかて)/せやけど(orそやけど) for そうだけど/それでも/そうだとしても/You're right, but.../You may be right, but.../You say so, but...
●せやし(or そやし) for だから/そうだから/so.../therefore...
●せやった(or そやった) for そうだった(the past tense of そうだ)
そやなandせやなin meaning, but there's an image ofせやなtending to be used more by people over 40 andそやなused by comparatively younger people according to this article. – cypher Jul 3 '12 at 4:46