I too had the idea of using SafeStart during a nitrite spike- it was during my cycle. I found it useless because a) you are not supposed to change the water for 7 days after adding and b) you cannot use an ammonia/nitrite detoxifying product such as Prime. It doesn't say so on the bottle, but Tetra's website states that if a detoxifying conditioner is used, you must wait 24 hours before adding the SafeStart. Nor can you add a detox conditioner at any point (they don't specify for exactly how long- I'm guessing 7 days) while SafeStart is 'working.' To me, that's just ridiculous- no way am I going to expose my fish to nitrite/ammonia poisoning so that I will have a quicker cycle.
I guess it's too late, since it sounds like you've already purchased the SafeStart- I know, I'm a big help.
But I would recommend Seachem's Stability for a cycle starter. You can use Prime and do water changes while using it, and it won't impair the product's effectiveness. My nitrite spike was over two days after using it, and my cycle complete. Whether this was due to Stability or simply my cycle's natural timeline is anyone's guess, however.