I ocassionally add some finely crushed eggshell, but a little garden soil would do, and I wouldn't really worry about grit in an established bin - the "average lifespan" isn't much to worry about either - if conditions aren't ideal they'll die, but leave cocoons that hatch when things improve ... 2-3" is adult, the 5" is usually older ones that have had ideal conditions all their lives ... if you're not "breeding worms" then as a waste disposal project, it doesn't matter

"lively" has to do with temperature as much as anything, unless they're really starving - the problem may be the compaction, which can end up making areas too wet or dry, and a lack of feed will slow reproduction and growth - ideally they like a looser bedding [you can fork it up once in awhile without bothering them], and when established they can use about 1/2 their weight per day in food

Bill