I don't have a Speedster, but they tend to be universal once you get down to 26" vs. 700c. However, there are also some traps / interference issues that might need to be addressed, e.g., V-brakes and/or a disc brake mount. Therefore, to get the BEST recommendation you'd want to be more specific about both your needs and your Speedster's configuration.
Needs: Will you use only a trunk bag or do you plan on using panniers? Or, will you use panniers and a trunk bag and what size panniers? How much "stuff" in terms of aprox total pounds will you be carrying on your racks? All racks have weight limits and they can range from 20 lbs (9kg) for a seatpost mounted rack, and from 40 lbs (18kg) to almost 90 lbs (40kg) for more traditional frame-mounted racks.
Budget: You can spend anywhere from $35 (Topeak) to $150 (Tubus) on a rack and from $35 to $300 on luggage.
Tandem: Does your Speedster have a rear disc or do you plan to install a rear disc in the future? Does you Speedster have the standard cantilever brake bosses and use V-brakes or does it have a caliper? Discs and V-brake can get in the way of come rack systems and that can limit your choices.
Just some food for thought. I'm sure if you sent an Email to
dwan@co-motion.com with the subject line: Best Rear Rack for Speedster? you'd get a pretty good recommendation within about a day or so.