Just reviving my own thread to perhaps help out anybody that may have my own question.
Although I cannot compare the same exact gauges, the very light RS77S set has indeed less tension than ecb81s, short scale chromes. But thankfully, not to flabby at all, I didn't need a truss rod adjustment, but did have to raise the bridge just a bit. Although all strings are technically higher, the bass is even easier to play. I do wish they had higher tension strings, but the Rotosound Jazz 77 sound fantastic. Bass tone from paradise.
As I said in another thread, I do like the Chromes, but I absolutely love the Rotosound flats, which is why I came to try them on my short scale bass. I have never found them repugnantly bright as many find them. Full sounding, but very articulate, matching the bass and Lollar pickups well.
Try these if you haven't on your shorties! They are not super tense, for better or worse. Great tone nonetheless, mind you. Even with the low tension and very low action, I didn't have problems playing softly or aggressively (I don't mind controlled "clack", as I even use it intentionally some times). I guess the usual stiffness of the monel strings make up a bit for the low tension. They don't flop around too much.
Summary:
-Looser than Chromes, but not by much. (Lighter tension on the Rotos for the short scale set explains why- perhaps if they were the same tension, the Rotosounds might feel stiffer than the Chromes, but I haven't tested it myself.)
-Less sustain with Rotos, but with richer tone quality (utterly subjective, but they have their own beautiful tonal quality which is why I came back to them. Chromes sound a bit honkier and nasal, but not in a horrid way; they also have their own voicing.) They (Chromes) probably have better upper mids, I am not sure, but they do sound different from each other, even if both are often considered bright-Rotos have a better perceived brightness (and openness, IMHO) overall, but neither set is muddy.
-Rotosounds are very full sounding, a detailed attack, but not a roundwound tone at all. Where is this zing? They just sing beautifully! Chromes are also not harsh at all. They are only bright compared to other flats.
Hope this might be of use to somebody in the future. This is not merely a "vs" thread, or "which is better"(the "vs" part was how did their tension compare for their short scale sets, which was my initial question), I did like the D'Addario Chromes short scale set, but leaned towards the great sounding Rotosound Jazz 77 flats, and it felt like going back home.