Fitting to MX size is simple. It's ebikes that are more difficult due to width issues. eg On my daily rider I draw 260A regularly through my string of Ford cells from Panasonic that have threaded posts as the terminals. I run 2 packs of those 31s1p of these 26ah nominal cells, each on daily commuters, and with the 10C peak discharge I've never noticed either pack getting warm. I've noticed a slight loss in capacity (maybe 10%) on one of the packs, but I was filling it up to 4.17V/cell twice a day for my commuter for a year and a half, and that pack was only seeing 180A peak with better air flow over the pack. The other 31s1p pack with cells from the same car sees higher peak and sustained current on a much heavier bike, and the cells have little-to-no cooling air flow, and that pack no has about the same mileage as the other, but very little capacity degradation. I attribute the better health to now using 4.05V/cell max charge, along with less frequent charging. That pack spends much more time in the 30%-90% SOC range, while the other was 50%-98% SOC. After reading that Chevy goes only slightly above 4V/cell, I top up with my chargers that go to 4.17V/cell for infrequent long rides.
Back when I did the combination chemistry packs good high power batteries really weren't available. A123's were expensive and hard to find for less than $8-10/cell for the 26650's. The only high power cells easy to get were the extremely dangerous and commonly faulty RC lipo packs. Good batteries are readily available now, so there's no reason to waste time and money on the over-rated in terms of power 18650's that are only useful in traction packs in the long range format Tesla uses, and even Tesla is apparently getting away from them. Sure they work fine in hybrid use on relatively low powered ebikes, but the power and structural integrity required for a MX emoto is another story.
In addition to cost, I have 2 problems with the combined chemistry battery idea. One is complexity, but the biggest issue I is whether it will even work as proposed. I'm not sure that without fully charging you can really get the LFP cells properly balanced. When I combined chemistries the LiIon portion of the pack carried the load early in the discharge. I question whether you can partially charge the LFP and have them carry the high power demands fresh off the charger. In the best case I think you'd have an unbalanced LFP pack, and worst case you'd kill the LiIon pack from getting hot due to bearing the brunt of the discharge load fresh off the charger.
FWIW, I don't care much for the motor selection, at least not for handling a 250A continuous input, 500A peak. Sure it can handle it, but the copper in the motor will get hot doing it despite the liquid cooling. The phase-to-phase resistance is .027ohms in a motor weighing 52lbs plus the weight and space used by the liquid cooling. The motors I use have a phase-to-phase resistance of .016ohms, and when pushed to 400A peak input there's plenty of heat to dump, and heat leads to lower efficiency and wasted battery capacity. You're talking about higher resistance (almost 70% higher) and higher current, not to mention having to carry around over 20lbs more even before the cooling system.
