As i say, they're initial thoughts and i don't claim to be an expert on the period! If you're interseted and haven't discovered it already there's a guy on iTunes called Mike duncan who does an excellent podcast called The History of Rome. It's free to subscribe and i really would recommend it if you like that sort of thing - it's really good to paint to if nothing else!
The principes were more experienced than the hastati, so although similarly armed, they are likely to be a little better equiped and arguably have better morale. So I would say you have it about right, maybe give the principes a similar armour save to the Triarii. The maniple rule you suggest might work, but for me it doesn't reflect the way they apparently operated. The Hastati charge and if they aren't victorious, they retire and the principes charge in. I reckon the rule should allow them to charge in and then should they draw or get a hold or withdraw in good order result on a breaktest then they retire behind the principes.Actually, using your idea of the maniple rule might be better if you allow the principes to counter charge if the hastati hold or withdraw in good order. If they withdraw disordered, then you could roll a dice for the counter charge - 4,5,6 and the prinipes manage it. anything else and they stand still. As for the cavalry, I think Mike might be right. Medium seems to fit the bill. At the time I think they were considered heavy cavalry because of the equipment, like the large shield, but I reckon Medium would be about right. Otherwise, make them heavy, but reflect their poor status by giving them a lower stamina value.
"Right! Open this door or I'll take away every piece of Warhammer you've ever owned!"