Ok, so I have 2 different orders - one client wants royal icing flowers on petitfours like on page 84 of wilton 2008 yearbook, the other like the cake on page 78...won't the flowers soften when the grease from the buttercream/fondant hits them? (they have in the past...)

they'll soften after some time, but they will still keep their shape.

Once royal icing is properly made and the flowers dried completely there should be little to no problems.
I don't have to book to see which flowers but any flower that has a thick base (ie:roses, even dropflowers) will be perfectly fine. Thinner flowers such as apple blossoms might pick up some grease but won't 'melt into a puddle'

Usually I have no problem putting royal icing flowers on a butter cream cake.. When I have had problems they did not get soft at all what happened was the color became darker

It was usually purple violts that were the problem and it was caused my putting them on the cake before it was crusted... I often smoothe with a hot wet knife and if is wet it will cause a problem

one other time my frosting was much too soft it had too much liquid in it so it is not the grease in the buttercream but the liquid... let it set to avoid problems

I use buttercream to attach the flowers but to avoid any problems use royal icing to make the leaves to attach the flowers an it will solve any problems