So you are not able to say how many of these different “denominations” have differing teachings. Different bodies do not necessarily mean different teachings. For example, due differences in times and methods of settlement in Canada and the existence of several colonies, there were four Presbyterian church organizations at the time of Confederation. Once Confederation made Canada a unified country, these four churches joined to form the Presbyterian Church in Canada.
In addition, if only difference between one denomination and another was that one did not allow instrumental music in church and the other did, would that be a significant difference in teaching?
This is quite unlikely to happen as if a minister was teaching unorthodox doctrines the Kirk Session would intervene and the presbytery that the congregation is in would investigate and could dismiss the minister. At worst, if presbytery did not act, members would likely move to a different church within the same denomination.
I can not really speak for the Baptists, but I expect that they also would have a method for dismissing a minister who would then likely look for a position in another already existing denomination that practices infant baptism.
Again you are making this statement without knowing the teachings of the different denominations and how they might actually differ from each other. For example when I lived in an area where there was no Presbyterian Church I had no problem attending a United Church of Canada even though I may disagree with some of their more liberal policies.
In Canada there is an Ecumenical Shared Ministries Task Force formed by the Anglican Church of Canada, the Evangelical Lutheran Church in Canada (which are in full communion with each other), the Presbyterian Church in Canada, and the United Church of Canada. This has resulted in sharing facilities, joint worship and sharing the same minister.