Here's what the song says will be taken (my emphasis):
“Come seek us where our voices sound,
We cannot sing above the ground,
And while you’re searching ponder this:
We’ve taken what you’ll sorely miss,
An hour long you’ll have to look,
And to recover what we took,
But past an hour — the prospect’s black,
Too late, it’s gone, it won’t come back.”
All this means is that the thing taken will be something that the champion will sorely miss. This is open to many possibilities. It doesn't even say that the thing will be a person. In fact, Harry seems to have thought it would be an object:
But Harry wasn’t listening; he was thinking about the merpeople’s
song again. “We’ve taken what you’ll sorely miss.” That sounded as
though they were going to steal something of his, something he had to
get back. What were they going to take?
And even when Ron and Hermione were mysteriously called away to McGonagall's office and never came back, he didn't suspect that they were taken for the second task.
Additionally, when Bagman tells the crowd what the Second Task consists of, he simply says that the champions have to recover what has been taken from them:
“Well, all our champions are ready for the second task, which will
start on my whistle. They have precisely an hour to recover what has
been taken from them. On the count of three, then. One ... two ...
three!”
When Dobby finds Harry he tells him what they've taken:
“Dobby knows, sir! Harry Potter has to go into the lake and find his
Wheezy — ”
“Find my what?”
“ — and take his Wheezy back from the merpeople!”
“What’s a Wheezy?”
“Your Wheezy, sir, your Wheezy — Wheezy who is giving Dobby his
sweater!”
Dobby plucked at the shrunken maroon sweater he was now wearing over
his shorts.
“What?” Harry gasped. “They’ve got ... they’ve got Ron?”
“The thing Harry Potter will miss most, sir!” squeaked Dobby. “ ‘But
past an hour — ’ ”
Dobby seems to have decided that the song was referring to what the champion would miss most. However, we have no reason to grant credence to Dobby's interpretation (and perhaps he was just stating the fact that indeed Ron was what Harry would miss most).
This being the case, it does not seem like there was any big need to figure out what to take. They could've taken anything that the champions would want back. Obviously, the more dear the object, the more intense the task would be, but if they could find something easily they might not even bother to expend more effort to find something better.
That Harry's, Cedric's, and Krum's loves were so openly known would be a lucky bonus for the tournament organizers. That they brought in Gabrielle for Fleur probably means that they couldn't easily find anything/anyone at Hogwarts to take from her1, or that Gabrielle was very easily accessible. (Alternatively, once they had taken a person for all the other champions they wanted to keep it consistent and take a person for Fleur too.)
In short, there may not have been any serious methodology by which the hostages were chosen, and the official clue from the egg and Bagman's announcement do not indicate that there should have been.
1. Even though Fleur had gone out with Roger Davies, it is possible that it wasn't very serious, or that they had already broken up. We know that by the Third Task Fleur was already eyeing Bill:
Fleur Delacour, Harry noticed, was eyeing Bill with great interest
over her mother’s shoulder. Harry could tell she had no objection
whatsoever to long hair or earrings with fangs on them.
And by Order of the Phoenix Davies already had a new girlfriend:
Roger Davies, the Ravenclaw Quidditch Captain, was sitting about a
foot and a half away with a pretty blonde girl. They were holding
hands.
