Like the lists and, as with the rules, the common sense rather than prescriptive approach, but 2 questions wondering if there was areason for this:
Hellenic Bactrian coinage shows use of the Kontos - my bodyguard cavalry will be - and, on the basis of the spread of Helenism to the Iranian aristocracy - Hellenic Bactria's aristocracy was an alliance of Greeks and Hellenised - Bactrians not just Greeks - so probably will the Bactrian cavalry.
Also why no cataphracts - the cataphract panoply found at Ai Khanoun and dating from 150-180 BC is near as dam it proof and is Hellenic in style
The Macedonian army of Alexander brought the Xyston into the east and there is no reason to state that Hellenistic Bactrian Cavalry did not use it in limited numbers. Fot King Eucratides the Great we have evidence of 300 sarissophoroi (lancer) cavalry but modern reconstruction is for armoured men on unarmoured horses. Most settlers in Bactria had been Greek, not Macedonian. These had no tradition of lancer cavalry.
Cataphract armour is shown in sculptural art but there are problems with the dating.
Alexandrian cavalry tactic was for a fast charge of the Macedon horse that decided the battle. But in Hellenistic armies that vigorous use gave way to more cautious tactic. Spear throwing horse replaced the charging one. In the east the large numbers of Archers became the main cavalry arm. A Cataphract is slower and hardly able to do it in Alexander's way. Hard hitting and fast.
You know - it's a while since I actually wrote the lists - so I can't easily put my finger on all the sources I used I'm afraid - but - as with all the lists - I took a view and -on the whole - I also tried to match players' collections in so far as I could.
HC or Cats - bear in mind horse armour does not equate literally to cataphract in HC - lots of barded or otherwise armoured horses are HC - it depends on mobility and tactical role - and HC seemed to do this better. But if you prefer Cats - no worries - as you say the lists aren't intended to stop folks making their own judgements based on their own understanding:) My own interpretation is to use Cats only for the very heaviest cavalry - not for standard asiatic steppe types with horse armour - for example.
Kontos - sure - I gave this to the Saka type in the same list - I interpreted the bulk of Greek cavalry over the broad period as having a more general armament - but the lists can accomodate the addition of kontos if you prefer - they are meant to be adaptable.