I'm going to try virtually the same thing as soon as I get a chance, except that I was planning on using a secondary.
That said, I don't think it will be terrible if you don't. My understanding is that the reasons for using a secondary (as opposed to adding it to the primary after the main fermentation is complete) when adding flavoring additions like this are:
1) If you put the flavorings in first and rack over them, they get mixed better.
2) It's less of a mess, especially if you are planning on washing and keeping the yeast (not as much of a factor if you are using a bag, which is probably a really good idea with coconut from what I have heard)
3) There is some concern that if the flavorings sit in the trub, less surface area will be exposed to the beer and you'll get less efficiency.
Now of course there are lots MORE good reasons to wait until after the main fermentation is done, whether you are leaving it in the primary or racking it. My understanding is that the big ones are 1) volatile aroma and flavor compounds won't get carried out by CO2, and 2) there is significantly reduced chance of infection adding flavorings like this to alcohol-y beer as opposed to bacteria-friendly wort.
ElevenBrewCo is right that buckets are cheap, and it would probably be better to do your primary in a bucket and rack it to a secondary. If for whatever reason you really don't want to do that, you should be fine with the way you said.
As far as the amount... I read a recipe from a guy claiming to get great coconut aroma and flavor, who used 22oz of toasted flaked coconut in a 10-gal batch. I'm going to do a 3-gal batch for my first attempt at this wacky idea, so I was going to add 8-10 oz or so. Of course, I won't be able to let you know how it went for a minimum of ~6 or 7 weeks, so don't ask me!