Probably. Arbiters are not data bearing, so are not really involve in replication. You might even be able to use the mongos intance for that purpose.
As for the index, I think you incur processing overhead by making it a hashed index. I think mongos will rehash your hash value before distributing it. I think you are better off having a normal index on your hash field or forgo the hash field altogether and make hash index on your userid, teamnumber fields. The former will save processing time but the latter will save space. Space saving is often saving time too as there is less data to move and to read/write.