new smoke detector - connecting solid to stranded questions
I want to replace my 15 year old smoke detectors. Inside each electrical box, there were four 14 gauge white and black wires - each bundle wire nutted together. Three of each color originated from romex style cable and the fourth was was used as a pigtail to wire to the back of the smoke detector (inserted to the back of the unit). A third red wire was wired directly to the back of the detector.
The new Kiddle detectors came with a flexible wiring harness using 16 or 18 gauge (can't quite read it) stranded wire (white, black and red) with pre-tinned ends.
My question is how to connect the stranded wire to the solid gauge wire. Should I leave the white and black pigtails as is and wire nut the white and black stranded wires to the pigtails or should I eliminate the pigtail and connect each stranded wire to the bundle of three 14 gauge solid wires? My limited experience with wire nutting stranded to a bundle of solids is that the wire nut never really tightens up without shredding the stranded wire. Also, obviously the bundle of solid wires are already twisted, so should I straighten them out (or cut off the twisted sections) before laying the stranded wire alongside and putting on the wire nut?
Thanks for any help.