Hello,
yes, we can stitch +ve edge followed -ve edge flops in same scan chain. It will not create any issues.
But you can not connect -ve edge followed by +ve edge flops as its violating the shift path that you can check by doing timing analysis.
Could you explain more please - why +ve edge followed by -ve edge flops will not create timing violations on the shift path while the -ve edge followed by +ve edge flops will do create such violation?
There are so many links available on internet. Can you please check there? Do google like : Lock up latch in DFT. Because It is quite difficult to understand here.
You can do small exercise like :
1. Draw a 4 scan flop sequence. Do timing analysis with waveforms.
Do like - if first 2 are +ve edge and last 2 are -ve edge and vice versa. If you do for shifting only, you will get better understanding.