I see ap-southeast-2(Sydney) has only “Query Group” fulfillment option in AWS Marketplace page. Does it mean that a query group can join an existing Production topology running in a different region? Can I use it as an effective cache for read operation?

I'm fairly certain that that wouldn't work, but I'd obviously defer to the Datomic folks. a system is encapsulated by a vpc so, I don't think there's currently any cross-region support

I have a datomic-peer application that runs/scale well using c5.xlarge machines If I buy datomic-ions, how do I get this machine? Should I buy m5 then change to c5 ? I will lose something by change the instance type? how easy is change it?

you can't, its not supported at least