Will replacing an 18 strand string with a 16 strand cause my tuning to change enough to cause arrow flight problems.

If its the same material very very doubtful.

I've swapped out a 16 for a 12-strand dacron string and didn't notice any change.

I have not seen much with a few strands in change with the same string material.  I have seen a noticeable difference with string material type.  And some big time changes with differences in silencer material or even just a little more or less silencer material of the same type.

Which type of material will have less stretch and creep, Astro flight or fast flight plus. I plan on making a 18 or 16 strand flemish string.

Of the two Astro Flight.  UltraCam has even less, With either string material you can go down to 8 or 10 strands with no problem.

Will I see any difference in arrow flight as I am changing from fast flight plus to astro flight. I know the astro flight is less prone to stretch and creep but will it change tuning enough for me to see it if I go from a 18 strand fast flight plus to a 16 strand astro flight string?

The best way to answer your question is for you to shoot a bare shaft with both strings. That will tell you for sure how your tune is.

I have heard some people say that all fast flight material is so similar that it does not make a difference in tuning between one or another as long as you keep the strand count within about 2. Is this correct?