Actually, BuffMePlz (the guy that makes bots) explained in a thread why you couldn't get to use Intermediate bots in custom games...
Basically, there is quite a bit of differences between Beginner and Intermediate, like masteries runes, "rubber band comeback", summoner spells... But the biggest of them all is interaction. In fact, beginner bots are 5x1, where a intermediates are 5. What I mean by it is that other than choosing lanes at the start, beginner bots won't make any team plays (although they may end up coordinated by coincidence). However, intermediate AI is made to coordinate themselves. They will roam/gank, they will sometimes group up and interact like that. Doing so requires "input actions", and AI bots basically have states which simplify that, but doing the same with humans would require to analyze everyone's actions to assess a situation, and from that situation decide an output. It is basically as complex as what already exists, so it will take another "Rise of the Bots" 'till we can see Intermediate in customs