1. Talking about "Partnership" in general, you can become a partner at any time, even at 0 subscribers. YouTube's own "Partner Program" which before used to have requirements and standards...but now, is given to everybody... you don't even have to "apply", you simply enable it in your channel settings at:
https://www.youtube.com/features and there you go, partnered....granted you need to create an AdSense account and link it to your YouTube in order to get paid.
The other type is "Network Partnership" where you apply to join a network and if accepted, you sign a contract, accept their CMS invitation and you're in... you'll often hear networks being called MCN's, with these programs you'll need a PayPal account and there are so many networks big and small in loads of different categories with different benefits.
You can get both types of partnership at 0 subs and 0 views, however certain networks still retain some requirements to join whether that be stat wise or content wise.
2. With YouTube you get scheduled uploads, custom thumbnails and "Official Series Playlists"...I believe that is all the benefits with partnering via YouTube there is left, everything else you can obtain by verifying with a phone number here:
http://youtube.com/verify
With Networks you get everything that YouTube offers except various networks can offer many other things as well, some usual examples are: No review for monetization (every network), A partner only forum, a custom dashboard, access to libraries of royalty free music, a support team via tickets or email, refer-a-friend link so you can earn from people you refer to the network, sponsorships, various applications to help with growth, SEO and analytics, Content ID protection against reuploads and various other things, those are the most common and every network has different options, however, 99% of MCN's take a percentage from your earnings you would usually get from YouTube alone and some require you to sign contracts to where you have to stay with them anywhere from a year to 3 years, this is to cover the benefits you get and to employ people and....well, their pockets.
3. That depends on yourself, it depends on what kind of content you produce, how serious you are about YouTube, whether you care about money or growing and the benefits you want out of it. I can bring in 50 people that will say it's worth it and I can also bring in 50 people that think it isn't based on their own experiences with various companies.
But the one thing that it doesn't do is hurt you in any way, becoming a partner isn't going to change anything for your audience except advertisements if you choose to enable them.
Hope this helped in some way!
Sorry.
. For Partnership Network like Machinima, Maker, Curse, etc. I think if you have good views or subs or have both of them 
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