Xenophon had some hoplites from Rhodes, which was a major centre of slinging skill. They will have joined up as hoplites for greater pay and glory, but they were able to convert to slingers when the need arose.

I would have left off the armour etc with the baggage train and gone as light as possible to facilitate a rapid exit once the main body of the 10,000 had successfully escaped. The whole point of using slingers in this engagement was to outrange the enemy archers, so getting hit should not be on the agenda.

Which is why he was specifically picking Rhodians, rather than taking 200 at random.