Hi there,
Just to put a BOW on this: The OVAL solder PADS are the WAY to Go! when soldering the Xiao to the PCB. (NOTE) I always do the BATT pads FIRST! If you flip the PCB and the Xiao doesn’t fall off , Your halfway there 
Then I Power it on. I use the RGB Flash Uf2 (drag & Drop file) from the testing thread on here Always, I never have a failure with this technique 
40 something Xiao’s on custom PCB’s (3) revisions)
Here is the final result of what they Built… Yea they sent it to me for the contribution to the topic. 
thanx Fellows, and ladie tech’s. 
Works really Well!
HTH
GL 
PJ