^^^^^ The MX88 is a medium stiff ski, not a plank but stiffer than many. I ski it in a 178, am 165 and 6'.
The 188 thing is, IMO, a mistaken conceptualization of weight and length. Resistance to flex is a mathematical function of the cross sectional area across whatever section of the ski you're worried about. Typically the middle, which is thickest. Period. Obviously, controlled for material; an I beam will be stiffer than a styrofoam copy. So at 210, you want a ski that's built with stiff materials, or really thick like the old VR17's, or some combination of both. Also true at a constant speed, since F = m x a, so if you ski fast, that'll increase the G's in the belly of the turn that the ski has to resist. The length of the ski is technically irrelevant; plenty of 165 WC SL's would make you happy because they're stiff as those I-beams. IMO you'd like a MX88 on the line because that's the stiffest part of the ski and gives you a bit more stability up front. One caveat: a few companies make their longest length extra stiff because the assumption is that it's the "athlete's model," for guys who do crazy speeds. But in general, as I understand it, each other length is designed to be the same stiffness.
Your height will determine how much force you need to exert against the front of the ski to pressure the tip during a turn. A problem of leverage. Length also obviously relates to stability, which is a measure of inertia, really. So a 178 could be entirely fine for a 300 lb guy who's 5' 9" and doesn't seriously rip. It could be short for a 150 lb guy who's 6' 2" and does rip.
The RX is a pretty stiff ski. Stiffer than the MX88 for sure. Two sheets of Titanal, one .5 and one .8. SJ has noted somewhere that the stiff tip, which is not very flared, can create issues for hooking up if it doesn't get enough pressure. I ski it in a 168. No issues with hook up. At 176, agree it'd be more GS-ish still, and I'd have to pay a bit more attention to really getting forward into the initiation. But more fun at speed in rutty snow. So an issue of how we each define recreation, not what's the "right" length.
You should be fine on a 178. Unless you use it north of 50 mph in variable snow...