We stick to rats, nothing above 250gms (feeding multiple items at one feed rather than one big whopper), as anything above that can be pretty fatty, we also feed on rabbits or quail.
Guinea pigs aren't really too fantastic as a steady diet.
All our snakes are healthy and happy. As far as too big for rats goes as I said before multiple items at a feed; we feed olives and BHPs this way and they're in prime condition.