Monstrous Charge knocks down anything with lower Strength, so Cave Troll yes, not Mordor Troll or Balrog. Where the rules are unclear (maybe someone else can interpret it better than me), is what they do with other models that have Monstrous Charge. The FAQ doesn't answer this.
Quote:
Additionally, this model will still get the Extra Attack bonus if it charges, or is subsequently charged by [...] another model with this special rule.
So, if a Dragon charges a Fellbeast, the Dragon gets its extra attack. If the Fellbeast also charged, it keeps the extra attack. The unclear bit is whether the Dragon gets to knock the Fellbeast down. In previous versions of the rule, each model had a separate entry with different wording on this: Fellbeasts couldn't be knocked down whether they charged or not, Shelob could only be knocked down if she didn't charge, and the Spider Queen could always be knocked down whether she charged or not.
The way I read it, it doesn't say anything about losing the bonus, so as long as your model charged and has higher Strength, it gets the knockdown. So, my ruling would be that a charging Strength 7 model can knock down a Cave Troll and a Fellbeast, not anything else on that list.