For an RP, I would buy RF lenses and not old EF lenses. EF, however, fits RF with an adapter. RF, however, does not fit EF. That's how it looks. Native lenses are always better. Adapters are stopgap solutions.
For the night sky and the Milky Way, I would recommend a fast wide-angle lens. For APS-C around 10-15mm. I don't know if there is anything for Canon. The Samyang 12mm with f2. is very popular for this. I don't know if it is also available for Canon.
For the Neowise something longer is recommended. This 50mm could be suitable if you don't want to spend a lot of money. Be careful that the exposure is not too long. I wouldn't go longer than 5-10s. But you will quickly notice when the stars around you turn into lines. Be sure to shoot in RAW. The old 550D could still exceed its limits.
I have both and I have to be honest, apart from the aperture, not really much.
But I prefer the 1.8 as it is very small.
I couldn't notice any significant difference in the image quality.
Yes, you can use any lens with an EF or EF-S mount type on the EOS 400D.
However, if you put an EF on the 400D, you have to convert the focal length. Canon's conversion factor for the so-called APS-C sensors is 1.6.
So the EF 50mm on the 400D effectively has a focal length of 80mm.
Lenses with EF-S mount are made exactly for APS-C sensors. You don't have to convert anything there either.