Problem is that after 2 years their growth gets stunted and their roots get constricted. At this point they will gradually die op stop growing altogether, and when planted out they often die due to transplant shock.
Species that are ''suitable'' for pot culture are :
1. eucalyptus vernicosa ( dwarf species)
2. eucalyptus cinerea
3. eucalyptus gunni ( dwarf cultivars preferred)
4. eucalyptus nicholii ( less suitable)
In my opinion keeping eucalyptus in pots is a waste of money and ecualypt. You can only grow them properly in Airpots, any other pot type will always lead to constricted roots and even still in airpot they will maybe live for 4 years. I only keep seedlings in pots for a few months ( usually from januari to april or may (and a occasional june) you have to plant them out young to prevent stunted growth or root damage.
planting them out too late will lead to unestablished eucs which are wind prone and reduce their ability to regenerate after a cold winter.
Im not sure what climate zone Toronto is but if you have enough snow in winter your eucs will live. Snow act like a insulator and if you have a high amount of snow the temperature underneath that snow usually 26 F.
Be sure to start them early so they can get some good growth before getting planted out. I usually sow eucs in januari on a sunny windowsill. When it's very cloudy i put them close under growlights ( i use LED T5 but regulat5 high output lamps could work as well)i transplant them outside end of april or middle of may.
If you live in a cold part of the country and/or snow is rare you might want to check a specialist nursery for good size eucalyptus in airpots. Almost all eucalyptus in nurseries are rootbound or in very small rectangular pots, these are throw away eucs because they always die.
Species you should begin with are parvula, urnigera, aggregata, eucalyptus pauciflora, subspecies niphophila and debeuzevillei. eucalyptus dalrympleana is very fast growing and usually regenerate from the lignotuber, thus can survive very low temperatures as a dieback shrub. The foliage of this species is very prone to frost and wind damage due to its fast growth ( it rarely goes dormant). In my opinion parvula and urnigera are the hardiest species, parvula is slow growing which make it better suited for gardens.
Be sure to protect your eucalyptus trunk with a burlap wrap and if you have alot of snow just burry the trunk with snow to insulate it.
Eucalyptus are easy to grow from seed, i never buy trees because they are so easy and fast growing from seeds. Be sure to buy seeds from a specialist which offer you quality seeds, most eucalyptus seeds don't have a long shelf life so good quality seeds are needed.
i recommend you read the information on growing eucalyptus thoroughly because it is very different than ordinary shrubs and trees.
Other australian plants like callistemon, calliandra, acacia, knightea, banksis - all protea related species can be grown in pots indoors. Prostanthera also does well indoors.
i wish you good luck with your endeavours