Would powerful nobles have their own courts at their personal castles, like the hightowers or redwynes or reynes would probably, but what about rowans or royces or brackens and blackwoods? what would those be like?

Absolutely. Indeed, we’ve seen examples of much smaller lords with their own courts: think about Lady Rohanne Webber, who has Septon Sefton and three septas, Maester Cerrick, Ser Lucas Inchfield her castellan, Lady Hellicent Uffering her former goodsister, a dozen knights and squires, and various pages. That’s a pretty significant household for a minor noble house. 

So for lords of the Rowans or Royces or Brackens or Blackwoods, imagine that but scaled up – add on the relatives of their vassals who might serve as pages, squires, castellans, masters-of-arms, stewards, ladies-in-waiting, add on the households of other noble families who’d married into the family (except much larger households than the occasional septon or unmarried sister, because one of the ways you demonstrate how awesome you are is to have a bigger retinue than anyone else), and so on and so forth.