A common myth about MMP is that it will lead to small parties holding the legislature hostage with unreasonable political power as their ransom. This has simply never been the case with MMP.
In all cases of a coalition government with MMP, there has never been a case where a small party has hijacked the legislature and been so uncooperative that they've threatened the life of the Parliament. Rather, small parties know that there is no use in trying to be uncooperative since defeating the government in hopes of winning a majority is near impossible under MMP.
Also, it is in a smaller party's best interest to cooperate since it is only thanks to their larger coalition partner that they have the opportunity to implement some of their policies. The larger coalition partner would also be hesitant in giving their junior partner too much power.
Which small parties could there be in Ontario under MMP?
It's difficult to answer this question looking at election results under our current system (First past the post). However, if we take the results of the last election (2003) we see that there in fact wouldn't have been so-called small parties; only the Liberals, Tories, and New Democrats would have been in the legislature. The Green Party won 2.8% of the vote in 2003 thereby falling short of the 3% threshold needed to elect MPPs to Queen's Park.