Again, it all depends on what style of riding you are doing.
For me, with any horse I work with, I start them in a rope halter/rope hackamore and make sure they know the basics.....lateral flextion, disengage the hind end, turn on the hauches, sideways, etc. Then I move to a single jointed snaffle, like an O-ring, D-ring, etc. but I do not try to teach collection with a bit like this. Like explained already, it has a nut-cracker effect that can really bother a lot of horses. I use a bit like this for Freestyle riding, like teaching my horse to stop without me using my reins, trail riding, teaching canter leads, stuff you can do on a loose rein. Then I'll move them up to a double jointed snaffle that has a link in the middle.....I prefer the JP by Korsteel oval mouth copper loose ring snaffle. I'll teach collection in this bit and really focus on longitudinal flexion.
Some horses HATE tongue and bar pressure. They'll chomp on the bit, toss their heads, put their tongue over the bit, have a busy mouth, invert, etc. so you may need to do some experimenting. I really like Myler bits, well most of them, so you may want to look into those. Even their snaffles can have maximum tongue relief for horses who like it and they also have a wide selection of curb bits if you ride western. Both of my horses need maximum tongue relief so I ride them in that mouthpiece. It's actually the Myler/Parelli cradle combo bit, the C3.
As for fitting I think the horse should learn to hold the bit, not have it cramed in the corners of his mouth. So I find the place where the bit would be too low and then raise it another notch so it's not hitting the teeth, and that's where I leave it. The horse will hold it in a comfortable position for him. This is with a snaffle, a curb bit should sit snuggly in the corners of the mouth.