I know this answer is a little late, but maybe it will help someone out there.
1) Yes, tenants can hire a Realtor.
2) the Realtor fee is a commission of 1 month's rent. If there are 2 Realtors [a listing realtor and buying realtor ( that is what the tenants agent is called) ], then the commission is split 50/50 between the 2 agents.
3) Who pays the commission varies dependent on the agreement the listing agent has with the landlord. There are 3 ways commission is paid. 1) Landlord pays both sides; 2) It is split between the tenant and landlord [ tenant pays his agent half a month's rent, and landlord pays his agent half month's rent]; or 3) Tenant pays both sides of the commission.
I hope this helps!