Yet, something tells me that Slughorn chose Ginny for her prodigious
Quidditch skills, not as a wizard,
Um, that seems to have come out of nowhere, I can't see what could have been taking place on the Hogwarts Express that would hint at Ginny's Quidditch talents. With all due respect, you seem to be proposing the theory and then asking us to find the supporting evidence for it!
The text says exactly why he chose her, good magic:
'How come you ended up in there, Ginny?'
'He saw me hex Zacharias Smith,' said Ginny, 'you remember that idiot from Hufflepuff who was in the DA? He kept on and on asking about what happened at the Ministry and in the end he annoyed me so much I hexed him - when Slughorn came in I thought I was going to get detention, but he just thought it was a really good hex and invited me to lunch! Mad, eh?'
Harry Potter and the Half-Blood Prince - p.141 - Bloomsbury - Chapter 7, The Slug Club
This seems to be supported by Slughorn's behaviour during the actual lunch:
'Oh dear!' chuckled Slughorn comfortably, looking round at Ginny who was glaring at Zabini around Slughorn's great belly. 'You want to be careful, Blaise! I saw this young lady perform the most marvellous Bat Bogey Hex as I was passing her carriage! I wouldn't cross her!'
Harry Potter and the Half-Blood Prince - p.139 - Bloomsbury - Chapter 7, The Slug Club
And this particular quote points to something else as well. Ginny is clearly quite feisty and presumably Slughorn saw that in her attack on Zacharias. Judging by his response - chuckling comfortably - to Ginny's outburst at Zabini, Slughorn rather enjoys that. Slughorn's whole thing is having a bit of an eye for a character, someone with a bit of charisma, a bit of a personality, a bit of spirit.
But also he makes mistakes, for example Marcus Belby. And with a lunch like this, he can afford to take a bit of a punt on someone like Ginny - in fact, that must surely be one of the means by which he collects the best people. He invites someone for lunch, sees how it goes; if it goes badly, they get missed out when the pies go round and don't seem to get invited back. But if it goes well, that's another box of crystallised pineapple.