credentials: I was an ornamental horticulture and landscape design
student and worker in the nineties but became disabled and had to
reluctantly change my career choice. my specialty and favorite part of
my job was ornamental pruning.
In the matter of the plantings:
It is hard to predict how large a specific tree might grow without
knowing the variables of the environment. Any number of things could
effect the growth of a tree; soil type and condition, pests, sun,
shade, the presence of other plants and trees, grass surrounding the
base, whether it is an arid or moist situation, temperatures, wind,
etc.. That being said these trees could grow to the height suggested
or even more if the environment is optimal for it to do so; or the
trees could be smaller and not reach more than 15 metres in that time
Also the trees may grow slowly for a number of years and have greater
growth in others making it hard to maintain a certain height.
I do know one thing regarding your situation. Pruning should not be
used to control the height of a tree if it can be avoided. Pruning is
often used to do this but in an ideal world the height of a tree
should not be controlled by pruning. The reality is that people often
find reasons to do such pruning because of a number of reasons. Most
often in an ornamental situation it is becuase the tree was the wrong
choice for the situation.
So, ideally, if there are valid concerns about the height of these
trees then yes they should be transplanted and another tree chosen for
this area. That is ideal.
One other solution is to keep trees in containers to control their
size. However in certain situations this can backfire and cause more
problems than it is worth. If the roots are vigorous for example they
can outgrow the container and if the container is a permanent part of
the landscape cause it to be destroyed. This can be seen in street
trees that push up and through concrete sidewalks. Also, container
bound trees aften require more pruning depending on the species to
control its growth and this might include root pruning.
Now I am assuming you are in warmer clime but as you know the
availablity of certain species and their varieties of plants are
limited. Without knowing where you are located exactly limits my
ability to suggest an alternative. The region where I am located is
the Pacific Northwest in the United States so my knowledge of tropical
and sub-tropical plants is limited.
One suggestion is the brachychiton bidwillii or 'little kurrajong'
which is a smaller form of the plant you have already planted.
Another suggestion might be Eucalyptus leucoxylon ssp megalocarpa,
otherwise known as the red flowering gum which has pink to red
flowers, dark green leaves and large fruit much like your current
tree. This makes a great specimen tree and its predicted growth is
5-10 meters and grows fairly quickly.
I hope this helps.