Is that generally the right idea?

In practice, that's generally the idea. Small parties haven't gone away because larger parties can usually bet on them lending a hand when the large party fails to win a majority, and voters aren't afraid to vote for a small party when polls are split. However, this is mostly a left wing thing, at least in Canada. Small right wing parties tend to eventually join up with the "big tent" Conservative party. Although it's mostly because small right wing parties tend to be unable to convince conservative voters to switch from the big party to their little party.

Because it's a unicameral legislature that appoints the executive, there's no possibility of checks and balances by bicameral legislature and pseudo-democratic election of executive.

Canada has a bicameral legislature, just like the UK. Our second chamber is the Senate, modelled after the UKs second chamber, The House of Lords. Senators are appointed by the Prime Minister and can serve until 75 (technically they are appointed by the reigning monarch but the constitution requires them to listen to the Prime Minister). All bills must pass a vote in both chambers before it is law.

In practice, the appointed Senators don't like to vote down bills that have been approved by the elected Members of Parliament since it upsets Canadians who have been asking "what is the point of the senate?" and "why don't we get rid of it?" for a long time. They will typically only request small changes to avoid loosing their very cushy jobs, though there are times they do play politics. They claim to be the chamber of "sober second thought", where things are debated on their merits without political fervor. To their credit, most of their debates do end with a unanimous decision.

Is it safe to assume that some things, like changing a federal or provincial constitution, would take more than a majority?

For changing the constitution, it requires approval of Parliament (technically the Senate has a say but at most they can only delay changes for 180 days) and 7 out of 10 provinces. In cases where the change affects only one province, only Parliament that province needs to approve.

There are a few special parts of the constitution that need to be absolutely unanimous: removal of the monarchy, lowering the minumum number of seats a province has in parliment, removing English or French from the offical languages, and changing the composition of the Supreme Court.