Hi Omid and welcome! You've found the one and only teaching system that understands the movements used by the world's best skiers (WC racers) and applies them to recreational skiing.
... the RTM86 feel like a one directional rail straight down the mountain. Lots of fun, but the moment I try to slow down to practice any drills, it's all fight.
No surprise. My old Volkyls skied just like that. Harald has consistently panned most of their models for being too stiff to turn except by rotating.
Important aside: abandon the idea that you're an intermediate skier. Unless you've done race training with a good coach, it's unlikely you're using even the lowest (Yellow) level PMTS movements. Before starting PMTS I called myself advanced (PSIA Level 8+, which is meaningless btw). In PMTS terms I was a barely a novice, as you likely are. The only way to progress in PMTS is to forget everything you think you know about skiing - because most of it is misapplied or just plain wrong. Like 99% of experienced skiers, my first day at PMTS camp was a revelation in embarrassment. I couldn't perform the simplest (looking) maneuvers, even after the coach demonstrated them perfectly. Once I set my ego aside and willingly abandoned old movements, progress could begin. Since then, the journey has been profoundly rewarding.
As to ski length, SL-style carvers like the i.SS Speed should generally be skied shorter than one's all mountain skis. A longer length makes it harder to learn PMTS movements and/or stay on course through SL gates.
When I was shopping for i.SS Speeds (2015-16) and skiing with Diana (a Black Level Coach), she debated 163 vs. 170 for me. I'm 5'10" but only 130lb (skinny). When she asked my weight, the answer tipped her toward 163. This has been a good length for me in this ski. Whether your 10% greater weight would tip the recommendation to 170 is hard to say. It's not all about weight or height. It's also about ability, which is impossible to assess via the internet unless you post video of your skiing. FWIW, Harald skis this model at 170. He's almost exactly your size but in terms of ability he's on a different planet. Due to his abilities, not his size, Harald can take a ski that you or I could barely manage, bend it into clean arcs and make it look easy. (sadly, it isn't!)
So... no definitive recommendation because there's no "perfect" length for any ski/skier combination. It somewhat depends on what you want. For fastest learning of PMTS movements, the 163 might have an edge over the 170. For stability at speed or on steep ice, the 170 would offer some advantage. However, if you're serious about learning PMTS you won't be skiing fast or on challenging terrain for the next couple of seasons, since both of these interfere with learning new movements.
Good luck and again, welcome!