I just noticed that my keyboard amp (Roland KC-550) includes an XLR input. Is it possible (or recommended) to input aux sends from the mixer through the keyboard. The KC-550 has 4 input channels and I normally DI my stage piano. I am particularly worried about the speaker-level / mic-level thing which might ruin my amp.
Yes, you can do this. The KC-550 amp is kind of designed to be the center of attention for the keyboardist -- you could actually use it AS your DI and monitor simultaneously, but then you wouldn't be able to monitor the rest of the mix. According to the owner's manual for the KC-550, the XLR input for channel 1 is the only balanced input on the amp, which makes it reasonable for accepting an aux send from your mixer.
The manual also states that any of the channel 1 inputs will accept mic level or line level. You should notice that notches 8-10 on the channel 1 level knob indicate additional boost for mic level. So, if you're taking a line level on that input, don't go past 8.
You mention speaker level/mic level as a cause for concern. "Speaker level" isn't really a thing -- if you are sending a power-amplifier output to anything other than a passive loudspeaker over dedicated heavy-gauge speaker cable, you are going to have a bad time. Anything going in and out of a snake from the stage to the house mixer is going to be line-level or lower. Putting a mic-level signal into a line level input is harmless, and doing the opposite on this amp should only cause clipping if you are not managing the input as previously discussed.
So, to summarize, if you are going from your keyboard -> DI -> house mixer -> line level aux send of full mix -> KC-550 balanced XLR input, just keep the channel 1 level below 8 and you'll be fine.
An alternative way of using your amp would be to plug the keyboard directly into channel 1, and use the balanced XLR line out to go to the house mixer (just make sure they know you're sending a line level instead of a mic level). If you wanted to also monitor an aux send from the mixer, you could use a "reverse DI" (balanced to unbalanced converter, like this) to take the balanced send into the unbalanced channel 4 input on your amp, and then set the channel 4 output select to setting "A" so that this channel is not included in the line out being sent back to the mixer. (Forgoing the use of the reverse DI box in this instance and just adapting from XLR to 1/4" would work, but the entire run of cable would become unbalanced and would be subject to interference. The reverse DI box allows the long run of cable to remain balanced when the input you're going to is unbalanced only.)
One thing you need to realize is that the keyboard speakers are designed to make you heard, to project whereas monitor speakers are designed to not make your monitoring output heard in direction of the audience. So going through the keyboard amp will likely easier result in interference with the sound from the PA for the audience, and will more likely lead to feedback.
So it's definitely possible that a monitoring solution using your keyboard speakers will work worse than a dedicated monitoring system.