Does the kit's recipe call for mango puree? Or is that just something you want to add?
Regardless, I'd just add it to the primary as long as it has the headspace remaining to handle any additional fermentation that might sometimes get things foamy on top again. Secondaries really don't provide the benefit return for the risk they add. Besides, if you are going to bottle carbonate your beer, that will be your secondary fermentation! There really is no benefit for a tertiary ferment. At least none I'm aware of.
If you are adding the puree to a recipe that doesn't call for it, then I wouldn't. I'd wait till I've done a few of the kits and had good results. Then I might try to see what fruit additions bring to the party in a later batch. However I've found that a lot of the fruit flavors that I desire in beer can be given by just the proper hop selection.
But it's your beer, so don't think I'm telling you what to do. I'm just stating what my consideration would be.
I must have made 4 or 5 batches of beer with grapefruit added in different ways. None really satisfactory. Then I made a IPA with just Cascade hops, and it had the best and strongest grapefruit taste and aroma of any of the previous beers that actually had grapefruit in them.
I believe there are some hop varieties that lend some of the mango flavor/aroma notes. But I've never looked specifically for them.