I can't speak to the stall question, but #25 chain is sufficient provided you follow a few simple rules:
- Make sure you have at least 180 degrees of wrap on each sprocket.
- Make sure your sprockets are properly aligned.
- Make sure the chain is properly tensioned.
- Make sure you buy quality chain - McMaster chain has been good to us. One year we bought less expensive chain from another supplier and had nothing but trouble with it. We will always use McMaster for chain now.
This year we had a high traction robot using #25 chain and had no issues. We've pushed robots sideways across the field. During our entire season, I only remember 3 robots who brought us to a stalemate while trying to push: 25, 68, and 296. There may have been others, but that's all I can recall. The moral of the story is despite doing lots of pushing against teams like these we had no issues with the #25 chain.