Hi Tom
The more experience you have the more appropriate it is to trust your instincts. But it is helpful to not sswitch around too much and not switch because what ytou are doing right now is just not working. When it gets difficult it may be helpful to stay with you practice.
That being said, adding some metta in the beginning can be very helpful. Give it a try and see for yourself. It is a good idea to do switches to amother technique intentionally, for example when you are on the breath and want to do s.t. else, decide to stay with the breath for, say, ten more breaths and then do the other thing. Be mindful of what happens during the switch and then set your new intention.
Alex