Is it legal for me to use Youtube's TOS as a base and modify it to represent my site better (I run a video site)
No it's not legal to use some's tos. Get down over http://www.elance.com and pay 20-100$ for a legal tos.
I don't see why you couldn't get someone else and modify to your own version if you have the same niche. Common now, everything is free but you just have to be a bit smarter to not get caught I am not advise you to do so but just telling
I believe as long as you reword a lot of the sentences and such in YouTube's TOS it is perfectly legal.
No, it's not perfectly legal to rewrite someone else's ToS. ToS, privacy policy, etc... are all subject to copyright. Depending on how much you reword it, it will probably still not be legal but it may be very difficult for them to prove (that is assuming they ever notice in the first place). Legal? No. Likely to get caught? Probably not. Cost if you do get caught? Way more than it is worth. Do a Google search. There are a number of free templates available. Some are even from lawyers.
Bluegrass make a good suggestion. There are lots of templates around, free and paid. It may seem like an easy option to copy someone copy but if you get caught its not worth the hassle. Imagine if you needed a new wallet, or a new watch, is it better to steal one or buy one? (Cleptos would probabl say steal) Copying other people work is the same as stealing no matter which way you reason it.