I had an Echo that I launched on a D20 booster with an E5-6 in the upper stage. The high thrust of the D got the rocket going well and then the long-burn E made for a nice show.
If FSI motors magically reappeared on the market, I think I'd go for the 21mm motors but stay away from the 27mm E and F motors. There are just too many composite motor options out there now that would be less prone to CATOs.
timmwood

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NAR 60014, Level 1, SAM 124