Parliamentarian veterans deployed into a RC country? Expect a worsening of relations when they start despoiling the local churches...
Actually its not as hair brained as you might think. In Flanders they fought alongside Royalist Catholic Frenchmen, and the Commonwealth already had an agreement with the Royalist Catholic Portugal that English traders would not be molested. English Protestants would be allowed to use their bible and bury their dead with Protestant Rites even on Catholic soil.