I took this question to Seachem some time back when it came up in a thread, and I was told that Prime and Flourish used together would cause the minerals in Flourish to be rendered useless to plants. Prime they said is effective in all respects for 36-48 hours. Their advice was to wait at least 24 hours before using any nutrient supplement, or to be absolutely certain, 48 hours.
They did also say that the heavy metal detoxifying of Prime is geared to trace amounts, such as would normally be encountered in tap water, and not significant levels of any metal which Prime is not able to deal with.
I dose Flurish the day folllowing the water change (I don't use Prime, but my conditioner does detoxify heavy metals). May be over cautious, but I assume Seachem know their products.