I don't see Heki ever becoming Shin's subordinate
Though I do expect him to be a deputy general under Shin for a campaign (s).
I see him being like Choutou. A solid dependable general capable of leading large amounts of men, mostly notable for his loyalty to the King.
Heki definitely has it in him to become a GG like Chou Tou, it's what I ultimately expect for him, but I can see him spending a few years as a member of the HSU. Like you agree, it is inevitable Shin will become his superior, he kinda already is in the pecking order, but we'll see.
I would prefer ROM to Heki, for sure.
Rokuomi joining the Hi Shin Unit as the "third" nicely parallels the Ouki army
It's only fitting for Shin to add an asset of the Ou Ki/Tou Army, considering he's already got assets from Mou Gou (So Sui), Duke Hyou (Gaku Rai, Ga Ro & Man Dou) and Kan Ki (Na Ki & Saki clan).
Shin also has yet to add an officer that has fought in the golden era of the warring states. ROM might have only fought in the final years of it, but he's still got rare experience.
I also think Denrimi is safe for this war. If he was going to die, I think Hara would have had Shibashou kill him earlier. Though he'll probably die eventually
Probably eventually, but right now I see him surviving. Perhaps he gets fully integrated in the Ou Sen Army as and San Shuu will be free to join the Gyoku Hou.
I suppose there's a possibility Shi Ryou joins the DRM Army or they form a new one together (named after Sou'Ou in some way), but I think it's more likely Shi Ryou joins the Gyoku Hou, which is more to my liking.
Had Mou Ten joined the Ou Sen Army like I think he should have, then Shi Ryou joining the Gaku Ka after Sou'Ou's passing would've made sense/matched too.