Vs Sol, you need to outpoke him with your normals while maintaining a safe distance (Slayer's normals are better than Sol's). VV can be a pain, so do not try to rush him down, unless you have tension for yrc/rrc, I guess. Always be prepared to anti-air him too; one mistimed Riot Stomp can make Sol eat a big CH off Slayer's 6P.
Ky has better normals and better projectiles, but a worse DP. So that means you can play more aggressively against him (and you should). A bad DP means that Ky is more vulnerable to mixups and oki's. And thankfully Slayer has good mixups and oki's.
Outside of in combos and oki's, you generally don't use dandy steps much unless you have meter for rc. If you do have meter, you can do some crazy stuff with dandy steps.
1. D-step into option select pilebunker/yrc;
2. Empty D-step into yrc just to mess with your opponent's mind;
3. D-step into helter skelter into yrc;
If you're at neutral and don't have meter, you may try to land Under Pressure off a dandy step if you feel gutsy enough. (If you can make your opponent block under pressure, it's a happy day for Slayer since his mixups off that move are crazy.) You may also try doing d-step into helter once in a while, since helter recovers fast.