Nothing wrong even in hindsight about wanting Puel gone, wanting him gone wasn't the issue (he really needed to go)... the issue was who replaced him and the club got that wrong!
He has had an assistant manager since August...
https://www.southamptonfc.com/news/2019-08-23/southampton-football-club-first-team-assistant-coach-richard-kitzbichler
How have you worked that out? When Romeu, Hojbjerg, Ward-Prowse & Armstrong are all fit (as they have been all season so far) Slattery isn't going to be in the matchday squad.
I doubt that story is true and is his way of covering up the real reason. At the time Mings was at he club Saints had some tiny players in similar age groups who weren't released.
"Too small" is a nonsense... is Messi too small? Was Maradona too small? etc
5 hours is a lot, never takes me anywhere near that long to do pre-season. If time is an issue, you can play the streamlined version FM touch which is simpler and quicker.