I don't think you can re-order the list in Proximo. It does provide keybindings for selecting the targets in the list. So you could do a quick select target /assist.

But here's a macro to fill up Promixo with your opponents instantly..

Code:
/tar a
/tar b
/tar c
/tar d
/tar e
/tar f
/tar g
/tar h
/tar i
/tar j
/tar k
/tar l
/tar m
/tar n
/tar o
/tar p
/tar q
/tar r
/tar s
/tar t
/tar u
/tar v
/tar w
/tar x
/tar y
/tar z
It doesn't work 100%, but works most of the time for regular teams.