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Rps? You mean rpc's. Um.... NO!
Controlling another person's rpc without their permission is godmodding, and it is not acceptable in the rp world. Unless a character has "mind control" or any other psycic abilities that would allow them to, but you still have to leave the controlled party room to say if they will be hypnotised or not, and also hypnosis isn't exactly forever there has to be a time or trigger when they wake up, and you're basically dealing with an independent being that most definitely will fight back, so that will cost on the part of the controlling character.

The only case you can "use" another rper's rpc is if they are inactive but are likely to come back. You can't say what they said, but you could suggest what they did for example, if the rper left in the middle of a fight the rpc most likely still stayed fighting how you cover for them depends on the circumstances, it has to flow with their prior circumstances, and it has to follow with the character.

But npc's that are not entirely significant to the rp yeah, you can use those but only if it's for a short time and you do not intend to use them afterwards.

BUT

If you meant someones actual RP, yes you can take over it if they have been absent for a very long time approximately a month would be appropriate.
Skeletons depend. Is it in a guild? Are you of a high enough rank in the members to edit the post's of even the owner? And then again it all comes down to who's running it and how.
If you take over an rp in the forums but have not recreated it into the threads then newcomers will simply have to post if the owner isn't active anymore. But if you recreate an rp in continuation having them pm you their profiles provide for you to stick in one post and keep all profiles in one locatable spot.

Sorry for that. I couldn't quite understand your first post quite well... and I had to take a shower or my mother would've been quite the banshee and so I posted.

Just read through the rules very carefully. Some RP creators have a habit of testing people by adding special stipulations for joining such as if you have to send a PM you have to add something like "<3" in the subject.