On Shabbos we had a number of guests and had different types of pastries arranged in a pattern on a plate.
I wanted to change the pattern and so I substituted every third pastry putting the removed pastry on a different plate (which was used as well).
On reflection, I was concerned there was borer. Is there a problem with arranging food on a plate if it means removing some items?
This is not an issue of borer, because from the sound of it there is no actual “mixture” involved. A number of pastries on a plate, where each of them stands on its own, is not a true “mixture,” and therefore there is no issue of borer involved.
Even if there would be a true “mixture,” for instance where there are many small pastries piled up together and mingled up, it will still be permitted to sort them, by taking the desired pastries and placing them on the correct plate etc. Although they won’t be eaten right away, having the plates nicely arranged on the table is itself an immediate use, much as sorting cutlery in the right place on the table, which is permitted (Ma’or Ha-Shabbos, p. 486, citing from Rav Shlomo Zalman Auerbach.