Hi Shamshad
Hope you have referred the following Datasheet.
https://meraki.cisco.com/lib/pdf/meraki_datasheet_sfp.pdf
I do not see any compatibility challenges for the following accessories with MS 250
MA-SFP-1GB-SX
MA-CBL-40G-3M
You do not need to buy SFP modules for stacking.
Number of stacking cables shall be equal to the number of switches you are planning to put in a stack.
You may also refer to following url
https://documentation.meraki.com/MS/Stacking/Switch_Stacks
Regards,
Ajit
AjitsNW@gmail.com
www.ajit.network