I doubt DL would add YYZ-LGA given WS has 8 737s a day on the route. If anything DL would add an LGA frequency to YUL and YOW and maybe open up a daily flight to YQB once their preclearance facility opens. Any of that would involve DL cutting something else at LGA.
If the LFs on the 8x daily WS YYZ-LGA flights are low (possible given the high number of 736 schedules and WS' apparent desire not to schedule the Q400), I could see WS transferring several of these 8x daily flights to DL who have more appropriate equipment (e.g. CR7/CR9/A220). If this is do-able.
That's a big assumption, especially given that YYZ-LGA is one of the biggest international routes on the planet. I'd say it's likely that they use the 600 because it's the right size to run frequency.
The Q400 wouldn't work on the route, Americans have an aversion to turboprops, WS using a Q400 would essentially render them uncompetitive.
You'll see WS put the -600, -700, and -800 on LGA. They've got a good feel for when to run each type; the -600 might do LGA on a midweek departure, or perhaps a weekend rotation, whereas the -800s would do it on the busier times like Monday morning or Friday afternoon / evening.
The connections they are able to offer through LGA to the DL network are incredible. Within the last couple of years, the two carriers have grown closer, and WS is learning to maximize the options for their customers from YYZ. That will only continue as the JV is finalized, and we'll see more shared cities seeing significant passenger connections (BOS, DTW, ATL, SLC, LAX).