It's not 100% clear what you mean but I'll explain. Baton Pass allows you to switch Pokemon but keep any stat boosts (i.e. moves/abilities that have increased stats by a number of stages).
Slow Start does not count as a stat boost (or stat reduction) so Baton Passing from a regular Pokemon to Regigigas does not negate SS, nor would Baton Passing from Regigigas to another Pokemon (if he could learn BP) decrease the second Pokemon's stats.
However, it is certainly possible to reduce the effect of Slow Start, by Baton Passing from a boosted Pokemon. For example if Ninjask has speed boosted up to +2 his speed is doubled. Baton Passing to Regigigas would give him normal speed as if there was no SS. But after 5 turns Slow Start would wear off and Regigigas would have double his usual speed!