1. On a forum RP, how long are the average posts you are supposed to make? A few lines, a boat load or it does not matter?
Doesn't really matter. Don't go extremely overboard though and write a complete essay of a 1k words on every post
But other than that it is pretty much ok really.
2. Is it allowed to use other people's character in your own post? Not for major stuff, but having them move or answer for example.
If it is small stuff, like maybe a simple reply (like a yes/no answer or such) or moving them a little since it would be strange otherwise, then there isn't much harm in it. Though it is usually always better to ask consensus, or at least leave a disclaimer in your post that you are willing to change if people don't agree with the actions you took for them.
As there might be some actions you take that might seem small, but would be against the character of a person.
Personally, I tend to avoid it and leave the part open where people can reply / do their things. That way I can move the story forward, but still leave an opening for the others. If I need to edit a post of mine due to the reply, so be it.
.
