MongoDB Production Amazon - 3 data nodes per replica set, or 2 data nodes plus a shared arbiter
Option 2 has no problems in theory; however the more limited redundancy may be a problem.


About the arbiters:


Yes, you can run multiple arbiters on a single lightweight machine. An arbiter is a very lightweight process, doing little but voting, so you can run multiple arbiters on a single box. Each one must be a separate mongod process.


About the redundancy:


If you have a 2-data-node replica set, and one of the data nodes fails, then the other one can function as primary (with the vote of the arbiter), so you have resilience there, but you no longer have redundancy.

Whether that is a severe problem or not depends on your ops, how quickly you can fix or replace the failed node. That depends on your monitoring capabilities, your ops staff competence and availability, etc; that's a decision you need to evaluate.