I take it these take the place of the inserts that come with a set of arrows and you glue them in, is this correct?
Yes.
It appears to me that if I use the brass inserts (discussed above) I have to pick one and glue it in, if I don't like it I'm stuck with it because it is glued in. Is this correct?
With the ones I posted the link too yes, but there are some made by Gold Tip that you can adjust the weight of, I think they where mentioned earlier in this thread.