You cannot assume that it is the same Isobel. Unless you have definite proof, you could end up spending ages tracing a family to which you have no connection. The fact that you cannot find the record of a marriage between her and Robert Fulton just means that the record was never made or does not survive. As you say, the couple were married, as the entries for the baptisms of their children make this clear. If any of the witnesses to the baptisms of the Fulton children were from the Montgomery family of Meikle Cloak, or if one of the children were called Hugh, it might be worth looking into, otherwise it would be mere speculation.