If you are likely to have to mate Peco rail ends to their Atlas equivalents, you'd be advised to use Atlas' joiners. I can make Peco joiners work, but it's a bear, and very hard on the tender finger tips if you don't do some filing of the larger Atlas ends. Mind you, I was mating Peco Streamline Code 83 turnouts to Code 100 Atlas rails, quite a stretch. But, you can always crimp the ends on the smaller tracks if you have to...or solder...which is really the best bet for excellent and reliable connectivity at joiners.