You will get the same two answers here so I don't see how this will help you. You have to get as much information as you can about your equipment and then make an informed decision. Nobody wants to tell you,"yes, use what you have" because even when everything is perfectly matched, you can still have problems (sometimes violently) when charging
batteries.
The most problems in the past have been when using eGo type chargers to charge smaller
batteries. I am not sure if 650mah
batteries can handle the same rate as your 900mah Riva. I would assume that they can, but I am not 100% sure and I can't tell you what decision to make. I only know what I would do in that situation.
BTW I receive my 1000mah Twist tomorrow (Monday was a holiday here) and I ordered the generic eGo USB rapid charger for it. I know that the 1000mah battery can handle it. (Fingers crossed)