Quick Attack Cancel.
Coming at a Pikachu from above is asking for death. If you have to I'd think Ike's down b would work a lot better for thunder, utilt and usmash.
As for edgeguarding, spacing is key. Remember, you outrange him, so stand far enough away so he can't get you with his attacks or you have enough time to retaliate if he tries. If he happens get up on the ledge, you should be able to get at him. Granted, Pikachu's already good edgeguard game has been beefed up even more in brawl. One thing in particular to look out for is his forward b. In melee, it was so cumbersome that it didnt help for much. Now, it's what I use to come in from my edgeguarding for anyone who thinks they've given themself enough space from my edge attacks but isnt far enough for me to make it on the ledge without punishment, and it covers quite a lot of distance. You can knock them before they reach you, but its a hard attack to guage.