Lowering password complexity requirements after introducing a 2nd factor for authentication has no benefits. If you increase security in one place, just to lower it in another place you're just wasting your time. And in the case that one of your users loses their token/smartcard/2nd factor to an attacker, that attacker would have a weaker system to attack, therefor raising his/her chances to breach the system*.
The goal of multi-factor authentication is to make it even harder/more expensive for an attacker to circumvent your authentication process.
So the "best practice" for 2FA in your case is to not lower the already established minimum requirements. A 2nd factor should be treated as an addition to the system's security, not as a substitution.
*If we set aside rate limiting, IDS/IPS, account lockout, etc. for a second.