I think the important point to note is that at 650 points this is a relatively small force. Some of the Seleucid armies would be huge - look at the battle report versus Romans in this forum and I think that is more the idea of what a historical sized force would look like, as well as Luke Ueda-Sarson's web page with a detailed OB (but for another set of rules

) I do see what you mean about the Galatians as Luke shows them as groups of 4 on each wing, although there is nothing to prevent you from doing this in a scenario game.
I have used seleucid with some success, and this list from memory at 800 points is
4TC
6 Slingers LF Average
8 Archers LF AVg (much better value in this list than any MF poor archers - I would avoid them)
12 Phalanx
12 Phalanx
8 Agyraspids (I thought 8 was the maximum...maybe not)
4 Agema Superior
4 Agema Superior
4 Companions Elite
2 Elephants
6 Theuro.. MF
6 Theuro.. MF
4 Skythian LH
4 LH with Javelin
Little short as the last list I used was illegal with another Skythian battlegroup

. You need the foot skirmishers to protect the pike from shooting, and they are usefull elsewhere if you don't have to use them as a screeen. This army is nice in period as not many can match the mounted and deal with the pike.
You might be better with the Thorakitai then the Theurphoroi, as mine being protected only have gotten the ***t shot out of them by skirmishers, but thems what I got painted.