Seishirou is in both Tokyo Babylon and X, yes XD so is Subaru. Kamui and Fuuma are in X only. Various other TRC cameo characters are in X as well.

I recommend TB highly, at least if you like the CLAMP treats thematics (it's pretty similar to XXXholic in terms of mood and thematics, for example). X is slightly not as good, but is still loads of fun in a more APPOCALYPSE OPERA way.

All you heard about Seishirou/Subaru is undoubtedly true. XD They're my favourite ship for a reason.