If you already own the generator I say go for it and try it out. If the generator is anywhere decent then it will regulate the voltage more than enough for a motor so you won't over power the motor with voltage. If the generator can't handle it you will not harm the motor unless you are able to stall it for an extended period of time; the generator shouldn't be harmed either it would be like trying to use a dead battery if it can't push out enough juice it just won't.
The reason I've notice that people suggest higher wattage ones is because they are typically built a little better and since they are capable of more wattage they respond to a peak significantly better, I'm going into peak discussion. Generators supply electricity on demand (even the power stations'), a household generator will produce approximately a few hundred watts at idle, as more items are plugged in/turned on demand rises and the generator senses it and runs harder. Since there isn't a preemptive notice about this, the generator will typically spike/peak higher than the demand and recovery back around what it needs to put out. A motor turning on with capacitors will typically be able to handle that lag, the capacitors will just charge up slower (almost not noticeable in most cases) and the capacitors work as rectifiers handling another other voltage changes during operation. Motors without capacitors also typically handle it but since they don't get a beginning zap from the capacitors you run a small chance of burning out your coils, imagine plugging a 220v motor under load to 110v source it will energize the coils but remain stalled.
If this is a more permanent installation (off grid scenario as opposed to hurricane prep, etc.) I recommend a separate motor starter kit - bad with naming here - before the motor or a heavy duty UPS/Battery system but for just when the power fails I think you'll be fine.
250 ft head for the well = about 125 psi. You typically run the local system at 60 psi, so 180-200 psi.
http://www.engineeringtoolbox.com/pumps-power-d_505.html
says 15 hp. If we use the usual 1000W power to produce 1 hp, that's 15 kW. Your Granddad's 25 kW generator wasn't overkill. (It's actually 745 w/hp, but you've got a bunch of inefficiencies nibbling at you too.)
Alternatives:
Newer pumps are considerably more efficient. Gould has a line that has a built in VFD (variable frequency drive) You feed it single phase, it converts it to 3 phase, and also adusts the speed of the pump so that it doesn't cycle nearly as often. Do they have 3 phase VFD? Never looked.
If the place was set up and working with 3phase, then keep it. If you are going to do a lot of repairing/converting to solar to reduce the generator run time, etc, then getting a largish single phase generator, and converting where needed to 3 ph using VFDs may save you a lot of aggravation.
With an 8 inch casing, you may be able to run a windmill pump as well as your electric one at the same time. If the windmill fills a lined dugout, your power bill is cut in half.
Generators don't loaf well. You want to avoid having a generator under less than half load -- the engine runs too cool, and you are using too much fuel chasing bits of metal around in cicles.
Having a generator house with temperature controlled louvers, or exhaust fan will help keep the engine at it's best operating temperature.
If you are really concerned with efficiency, then have several units of various sizes, that cut in and out to match the demand. This is a difficult control set up, well beyond my expertise.
I would consider consulting a power engineer for this setup. If you have to replace the generator, and are thinking about the pump, then a few hundred bucks for a professional opinion may save you a lot of time and money.
Meanwhile, you can get a feel for how things are really operating by renting a 25 kW 3ph generator, and put a recorder to figure out how much of the time it's running, and how much power it's using. An engineer will want this information anyway.
Or Buy the generator at an equipment auction, if you need to resize later, sell it -- probably get what you paid for it.
Best Answer
I'm with Billy, I see no problem. Here are my concerns: You are not really needing to break R from Y2. You are breaking Y2 from Y2. The red will stay where it is to power all other features. And why is the heat pump second stage? Unless the reversing valve is stage one. And what is the back-up heat? If it is electric resistance, that will require a whole lot more power then the heat pump. That would be where you would want to shed load. With 22-KW of generator, what is the tonage of the heat pump. 22-KW can start a lot of heat pump.