You can bump the limits, yes, but IIRC asteroids get the full-fledged ship state check. There's a fairly well known bug that can occur if you've got too many things onscreen where the newest objects don't get collission detection, typically this means you lasers and it's quite annoying when your guns pass right through their target. It's been known to happen more in dense asteroid missions, much like the one you're trying to make. Don't let that stop you; just keep it in mind and don't go overboard. You might find that double or less the max amount allowed in FRED is fine, so try that before you put in a 10,000 rock field.