Is a peak hour train normally only one carriage?
No. That train being short-formed would have been a result of the disruption to service caused by trains failing in the heat on Wednesday.
Are Belair trains often late/slow?
It is to be expected on hot days, but can happen at any time thanks to the single line route and the lack of resilience in the system. All it needs is one person on a big fatty convenience tank
"mobility scooter" to make a clumsy 37 point turn (as opposed to a genuine wheelchair which can rotate on the spot) when getting off the train and the delays can spiral out of control for hours if the controllers aren't on the ball.
However, on Adelaide Metro a train has to be six complete minutes late before it is officially considered to not be on time - so never plan a connection shorter than eight minutes. This is a margin you won't find in any first world countries, and ironically also a bigger margin than the higher standards demanded (and kept) on the Adelaide Metro bus contracts despite the buses being subject to the congestion caused by Adelaide's 1960s-like car dependency.
The correct course of action when delayed over the 6 minute threshold is to complain to the passenger transport ombudsman and make use of your refund rights ... oh wait, we don't have either!
Are there often two services parked on the same platform at ARS?
Yes, quite often during the peaks but not uncommon at other times. Unless it's as a result of disruption and they are struggling to keep up with things, there should be plenty of announcements over the PA that "the 4:46 Belair train will be departing from the yard, or far, end of platform 5."