FX Impact M3 - Do I need a heavy liner?

I have a M3 with 500mm .22 STX Superior liner that I'm going to start shooting 23g slugs with. I had a few questions.

1) With the short 500mm length would I not need to bother with the STX Heavy liner?
2) I see guys mentioning "slug liner", What exactly makes a liner a slug liner?
3) Am I overthinking it and just go ahead and shoot with that stock STX liner?

Any input would be appreciated.
 
Does that apply to every calibers? Or can you shoot heavier slugs in bigger cals? (In standard liner)
I have .25 Maverick VP (600mm STX Superior liner) and it will work up to 33-34gr slugs (I have not found slug that will be as accurate as JSB/FX pellets in shorter range tough). Twist rate is not optimal for heavier stuff.