The one exception I might make to colonial builds being infantry only would be that Canada can build one transport. This would count as 2 towards the colonial maximum of 3 for that turn.
Perhaps, though, that must mean bringing Canada into the “even builds” rule; so that if the UK wants to build more than 4 infantry on a turn one of them must be in Canada.
It seems a fair way of representing British dependency on Trans-Atlantic shipping.