I hate to be the naysayer here, but i really don't think sub for sub does anybody any favours. Sure, your subs count will look good. But you won't be getting the views to tally up and it'll count against you in Youtube's algorithmic elastictrickery. Plus it'll be frustrating to think you're doing well but the numbers are inflated due to back scratching arrangements.
I have a second twitter account (different, i know) that i 'pumped' the followers up to 11,000 with followbacks and cyclical follower sprees followed by unfollow culls. It looked great and people imagined i was super popular. But NOBODY replied or interacted with my activity with any regularity. It was a great big empty balloon of an account!
I've left it alone now and in a year 5000 followers have gradually slipped away. And even with 6000 left i still have more fun and reach more people with my
@AndyManCam account which has less than 200 followers!
What I'm really trying to say is just keep making good videos and let people subscribe because they want to watch and they'll actively build your channel for you!
Please now feel entirely free to tell me to eff off and mind m' bizznizz!