I'm pretty sure you can only retain salary on 2 - 3 players at any one time.

1. Why would you make that trade with STL? What do you gain? Parayko? I would rather stick with Zaitsev and trade Riemsdyk for Vatanen or something!?
2. Florida wont make that trade, as they would have to protect Carrick.
3. Why would you want Despres (LHD)?

You're right, just checked to find out that max is 3 contracts at any given time. https://www.theglobeandmail.com/sports/hockey/globe-on-hockey/how-the-nhls-new-salary-trading-system-works/article7033878/

That's a terrible trade for Toronto in your STL deal.

Parayko is a significant upgrade over Zaitsev. He's two years younger, 6'-6", has big shot, and already established himself as stud on defence. While Zaitsev is very raw (not like there's a high ceiling), there could be another couple years of growing pains (see Gardiner). Plus, he may not even work out as hoped, and he carries a risky 7 year contract at 4.5M/yr until he's 32. Perhaps, I overpaid for Parayko and maybe the Leafs could get more in return, but St. Louis really values him. The only reason why I think the Leafs could pry Parayko away from St. Louis is because they have a lot of bad contracts, are tight against the cap, have desperate need for secondary scoring and centre, and (given the age of their star players) I think their window to win a cup is closing soon.

Assuming Florida will protect three defenceman, it will be Ekblad, Yandle (NMC), Demers (M-NMC). Its been confirmed that Yandle hasn't been asked to waive his contract, not sure if they can peddle Demers. Even if they were able to get rid of Demers, then they can only protect one of Mark Pysyk or Alex Petrovic. Hence, at least one of them if not both of them should be available. Sure Carrick would get exposed but he's far less likely to be claimed. I think Carrick is overrated, he's awful on defence and frequent liability, sure he can move the puck, but provides little offence at the NHL level, long story short, it's not that hard to find another Carrick in the league. Thus I believe Florida would ideally like to move one of Pysyk or Petrovic for a roster player that doesn't require protection but that's hard to find or at least package of prospects and picks.

Despres is a big, reliable, physical defenceman who can move the puck and has a wealth of playoff experience. Sure he's left handed defenceman but the price to acquire him should be cheaper than Vatanen or Manson and the Leafs could always just sign Franson (RHD). Marincin could get claimed by Vegas anyways. So the 3rd pairing could be (Despres/Franson). I think Despres becomes expendable given the Duck's influx of young top defenceman ready to break into the NHL and although Perron's cap hit is similar his contract expires after next season.