The weight I think should be fine. When it comes to rapid transit vehicles the terms "light" and "heavy" can be a bit misleading. The T1 subway and Line 3 MkI have similar weights, with the T1 actually being lighter per sq metre. Where weight would become a problem would be with standard LRVs - vehicles overbuilt with impact standards for operating in traffic. The Flexity vehicles are like tanks, and would no doubt pose a weight issue. Which I guess is why the Transit City SRT plan involved tearing down the elevated guideway, on top of modifying every station to have low platforms.
The train width could be slightly wider than the current Line 3 fleet (2.5m), but definitely not as wide as our massive subways (>3m). So perhaps in the 2.7m range, or with the same 2.5m floor width but 15-20cm wider at seat height. Track curves may be an issue, but the worst curves would be gone (Kennedy loop, Ellesmere tunnel). Either way a new train order could theoretically have greater articulation points as part its design.
Much of this was studied in the past, so it doesn't seem all that unfeasible to operate different trains. One thing I do wonder is whether the guideway can be fitted with third rail easily. But really even if we did decide to use conventional Toronto subways, building a new guideway and stations along the same alignment would probably be a fraction of the cost/time of a brand new all-underground design spec'd to 150m.
