Q1. To stack two MS425 switches, do I need two MA-CBL-40G-50CM cables?
Correct if you want to stack them using the QSFP+ ports, which you don't have to, you can go into the port configuration page and change any of the ports from Ethernet ports to Stack ports. MS425 supports "flexible stacking" so any front panel port can be used for stacking. Check for more info here in the install guide: https://documentation.meraki.com/MS/Installation_Guides/MS425_Series_Installation_Guide#Bringing_you...
Q2. To stack three MS225 switches, do I need three MA-CBL-40G-50CM cables?
Correct, OR the 1M or 3M stack cables.
Q3. To stack two MS425 switches, do I have to use two QSFP ports from Front panel?
Not necessarily, same answer as Q1.
Q4a. To stack three MS225 switches, do I have to use two QSFP ports from Back panel?
Correct, the MS225 has dedicated back-panel QSFP+ stacking ports, use MA-CBL-40G-xxx cables.
Q4b. Do all MS225 models have two QSFP Stack ports at the Back panel?
Yes
Q5. In three MS225 switches, one switch is MS225-48FP. Can PoE MS225 switch stack with Non-PoE MS225 Switches?
Yes, any MS225 can stack with any other MS225 and/or cross-stack with any other MS210.