A trailer with a 100# capacity fully loaded over the axle is unlikely to cause significant damage to your bicycle frame--even using the attachment device shown for the Rhode Gear trailer, above. The "tongue weight" of such a trailer would probably be in the twenty-to-thirty pound range.
Just don't load this trailer--or any trailer--with all of the weight forward of the axle. Minimize your tongue weight.
Remember we're talking about tubular steel on your bike which is some pretty strong stuff. I wouldn't hang an automobile trailer on it or anything, but it can handle this little trailer.
Also. How many times can you load a Rhode Gear trailer up with 100# and still have a trailer? I think the trailer is the weak link in this equation.
Most often, you will use this trailer with maybe 40-50# max, right? Even pulling it every day--not likely to cause much problem. If you start a bicycle-based concrete business, go back to the commuter bike.
I ride a Sakae Ringyo Litage. Thin aluminum tubing--road racing geometry--from the early 90's. I pull a trailer with it from time to time--sometimes a Burley flatbed with a lawnmower onboard. (Ususally, if I'm smart, I pull it with my Schwinn Tempo). Someday that aluminum frame is going to break on me. It isn't going to be the trailer's fault--it's going to be the 200# rider that causes it to break!
Your steel frame is going to be fine pulling that trailer for the trailer's entire natural life.