[Silfir]

Infielders with decent fielding ratings are usually designated SS by the AI (similarly, outfielders with good Range are generally designated CF), but that doesn't mean that's their only or even best position - it's just that it's the most difficult, and therefore most valuable infield position. Just have a look at their fielding ratings to see where they can play; if the fielding ratings are compatible position experience accumulates quickly.

The AI doesn't really understand this, however - you may have to manually insert them into these positions for a little while, so the AI manager takes notice of them as an option in that position.

In your four shortstops example, you make no mention of what was going on with second base or third base, for example. Any player with suitable ratings for shortstop can play anywhere on the infield. Or DH, or first base - literally anyone can play first base. As long as they're getting at-bats, it doesn't really matter where they play during defense, after all.