Are you talking about the Caribsea bags? If so, does it say "cuts down cycling time significantly" or does it actually say "no need to cycle?"
Either way, I believe using the LS, especially when coupled with LR, is capable of knocking a few days or maybe weeks off your cycling time.
However, if you're doing a reef, I would recommend letting the full cycle period pass regardless of what the bag says.
This may tie into my next point.
Some people say it's beneficial to lay down your DSB and LR, then keep the lights off in the tank for a full quarter. This gives the bacteria, detrivores, critters, etc time to breed and really seed the tank with nice life.
I don't know if this is so necessary, but my point is, you may want to put down your DSB and LR then keep the lights off for the full cycle period. Not a quarter. This might really help your tank get established.
I'm not too sure about the effectiveness of this, some mods or other member might know.
But either way, I would at least be patient with the cycling. A reef is a long-term hobby, investment, that you want to start as carefully and precisely as possible. Don't dance around tragedy to cut corners.
PS... I just noticed the last part of your question.
The ammonia spike depends on how much LR and LS you add. If it is a significant amount (not sure if by "have" you mean in the tank, or have to put in the tank) , then yes, as some life dies off it is very possible. Especially if you don't have an already established system that's capable of breaking all that nastiness down.
ALSO, you could put a hold on, or buy that fish and wait until the time is right. If the LFS scares you with its storing capabilities, I don't think those fish are too hard to find.
