There IS a minimum RPM floor on all models for alerting, and the 7000's will refuse to boot. (The 6000's will just yell at you.) The factory fan for the ICX7250 is most likely the same W40S or is 'very similar.' The RPM window is tuned somewhat broadly so that Brocade can change fan suppliers if necessary without needing to have fan-specific software.
And the W40S is why I would KILL to be able to change the FAN RPM expectations in the OS.
The Nidec W40S is rated for 25,000RPM at 12V with a peak that could reach 29,000RPM. It is not just 'high speed.' It was literally the highest speed 40mm fan period for quite some time. Sanyo Denki just introduced the 9HVA at 38,000RPM this year. (We don't count the 9CRH I use because it's a counter-rotating fan.)
But at 25,000RPM, we have options. The Nidec W40S, SanAce 9GAX, and Delta FFB0412. So we can reasonably assume the engineers tuned the window to accommodate these three fans. Especially as we have already observed the SanAce 9GAX in the ICX. This means that an ICX originally equipped with 'screamers' will have a VERY specific RPM window - a minimum of 11,500+-500 (or 1000) at 6V, and an overspeed trigger at around 26,500RPM.
Mind you, I am basing my assumption here on the 7250 using substantially the same fan as the 6450. Which would make the most sense from a sourcing, manufacturing, and code stability standpoint. (7450's and others with fan modules, totally different expectations. Especially as they use counter-rotating fans.) So 1 or 3 fans with 25kRPM @ 12V, and you need to hit 11kRPM at 6V.
Unfortunately that means the ONLY way you can get there is 1) faking the RPMs completely 2) connecting to a constant 12V supply with a 12kRPM fan and hoping it never asks for speed 2. Speed 2 will go into fan failure mode immediately.
This also, by the way, informs us as to what signals a tach faker needs to put out and what components. And it is a doozy - you need 50KHz square wave at about +-2.5% to prevent it reading fan failure.