Okay, some things:
Again, the shtick of Taldor is that the royal caste is bloated with nobility. I'd really like people not to go "but the map!" if you want to go but the map and use it to say we should ignore shtick, you really shouldn't be playing around in fantasy, where again, even Westeros, the purported gritty and realiest-est of fantasy boils down to complete and total nonsense. You kind of have to let some things go.
On Houses and social strata generally:
As I said earlier, the Senatorial caste are nobles. They're part of the bearded. They're hereditary nobility that the Grand Prince can also elevate people to, they are just lesser nobility, with lesser lands/wealth/powers.
The difference between them and a royal caste house/family is that the Grand Princes come from the royal caste, and are picked out/adopted by such from one if he has no heirs for instance. The Royal Caste are also well richer, and higher in prestige. They are the greater nobility. They have the more powerful titles and powers. While the Grand Prince can also raise people up to the royal caste, he does so more rarely. The main thing the Senate has against that is that they control, well, the Senate, and thus the imperial bureaucracy that runs a lot of things in Taldor. They don't, for instance, have private armies like a really powerful high noble family could.
While a royal caste family /can/ produce a Grand Prince, not all of them have. They just are the legal source of doing so.
And yes, a house could be made by splitting off another house. As long as everyone is of royal caste blood, it's really not much different than a change in the letterhead really.