I would have little difference between hastati and principes. At the scale that warmaster is designed to be play on I wouldn't put much in the way of difference between them and the triarii either, honestly. Maybe you could give them a 5+ save instead of a 6+ save for them having a little more staying power but that is tough because it means they actually save twice as many hits.
I think that the manipular formations would best be modeled with having small brigades of two units in each of the rows of checkerboard.
So have a front rank with 4 brigades of two units each, a second rank lined up exactly behind the spaces that you leave for them with four more brigades and then have a rear rank of triarii with 2 brigades.
The advantage this would offer is that an army attacking would push the first units back if victorious but if they are, say 60mm frontage with 60mm spaces and the next rank is 40mm behind them then they would only be able to push them 40mm back before they are lined up with the full rank. If they tie at some point then you have another unit ready to charge in on initiative in the next turn, if they lose then they are pushed back and at some point become level with the next rank, who because of how they are arranged will be able to bring some stands to bear into the combat.
xxx xxx
xxx xxx xxx xxx
xxx xxx xxx xxx
So if you have them arranged like that, with the bottom being the front of your army you can see how they would have some flexibility built in. You also deny the enemy the chance to use their numbers to great advantage if they decide to come en mass.
This has it's drawbacks, of course, you could be flanked as your army is obviously going to be deeper than your enemies but also not as wide.
But you have some serious ability to respond to that with the rear ranks fanning out if you have the command to do it.
Well worth a shot methinks.
I'd keep all the legion troops as being more or less the same, if anything maybe allow the triarii a bonus to receive commands, like they can reroll their first failed command in a turn if they are the first unit that commander has ordered or something like that.
fightingwise, at that scale, there would not be much difference.