Company is about 1000 - 3000, with a dev team of about 9.
The probation period is my main worry. The project hinges a lot on me and him working well together, so I don't want to make that not work, or make it struggle.
There's usually a pretty solid hierarchy in UK companies, at least from a development side. You have the Junior - Mid-Level - Senior progression. It was the same at my last place (I was actually a Senior on the job role) where you have Juniors under the Mid-Level and Mid-Level under the Senior.
I always listened to other developers though, I saw the role less as a "I'm the boss" position and just that I have more responsibility for what I'm doing. If I didn't listen to some of the Juniors (who haven't had the time to gain some bad habits :) ), a lot of good things would have been missed.