False.
The Cuba I have been recommending spans 500 years. I have suggested a modern leader (for obvious reasons) but I have also suggested plenty of non-modern leaders too.
As for the rest; yes Australia is big and yes it is a well off modern country but what else? Its history is not particularly interesting and there's no stand out leader I'd consider particularly great or at least well known. I doubt most Australians would even have an idea of who to pick as the country's leader.
Australia is nothing more than a well off former colony with a very uninteresting history with very uninteresting leaders.
I can not speak for Canada as I know very little of it.
Now again, I can't help but find it particularly bizarre that people (such as yourself) are more than happy to suggest plenty of other obscure civilizations with much shorter histories but the idea of including Cuba is unthinkable.
Castro is known mainly because the US hated him. Menzies is fairly well known, Barton, Chifley - Hawke would be the perfect choice for Australia but he's still very much alive. Within western economic circles Hawke/Keating is the yardstick in terms of getting it right. Australia cruised through the recent financial disaster because of the systems Hawke and Keating put in. The economies who followed that model survived well in the recent disaster -those that went the US route didn't. Other than size the four thing s Australia has that make them unique are
1) Unique resources - Opal & Marsupials come to mind but there are others (they also have tons of the traditional resources)
2) Aboriginal culture - if intergrated into the design they offer some unique flavoring
3) Convict culture - Australia started as a series of British penal colonies which no other major country did. That offers some totally unique design choices if devs choose to use them.
4) CSIRO - a government sponsored science org that developed a large number of useful developments -especially in medical sciences but also with things like the wi-fi many of us are probably using now for instance. Miltiary science wise both the Tank and torpedo were designed by Austalians. Australia is not well known as a science powerhouse but when you look at the list of inventions they are similar to the US in that innovations seem to have come out of the ex British colonials at a greater rate than you often know. To me the US's UA shold always have been science based because those guys are churning new stuff out at a amazing rate.
Canada is sometimes in the shadow of the US because of the US's cultural effect on the world. A decent list of science achievements most notably Insulin. It's probably not as unique as Australia in that they share the landmass so the American native parts are not unique to them. I'm sure someone with a better knowledge of them could make a better case for them than I can.
One thing people need to remember when considering Civs is not necessarily their size, relevance or success but rather what elements can be used in a civ development to make the civ interesting and hopefully unique. Cuba is kind of interesting as a place but what is their unique ability, what is their unique building? what is their unique unit? etc etc. Australia is by no means a perfect choice as a civ but they do offer some very clear possibilities design wise. I'm sure they could come up with things for Cuba but they are not as immediately obvious.
I dont think its unthinkable that Cuba could be considered I just think that as long as Castro is alive they dont really have the iconic leader and that he is the major selling point for them as a civ in civ5 terms. Without him they are a small island based culture with minimal obvious design choices. Castro dies and that changes because he's so iconic that the rest can be cobbled together around him. Of course thats what happened with Attila - they had an iconic leader and they cobbled together a civ around his idea.