Can a 25U be put into a 25W 1SG position?

MOS mismatch at the 1SG level is discouraged but not prohibited. In the case you mention both are in the same career field 25 so not as big a deal as if someone from say 12 series was being considered for a 25 series slot.
I personally have slotted folks for 1SG positions as MOS mismatches within the same career fields for deployments where no Inbounds were projected. Also have slotted folks as career field mismatches for the same reason.
As a PVT the unit I served in had an Infantryman serving as our 1SG in an Engineer unit, no issues other than not asking him to show me how to lay block or do concrete. All the other First Sergeant stuff was fine. Unless it's changed the AR was 600-20 and 600-200.

While it's preferred to match the MOS sometimes a mismatch can't be helped. I've been slotted against an 11B 1SG position for the past 8 months as a 25W.

1SG(P) (Join to see) Yes its can and does happen and IS supported by regulation. Though not supposed to be the better plan, nor done commonly. And there are restrictions and requirements to assign a 1SG position outside ones PMOS.

See AR 614-200
"When possible, the 1SG will possess the PMOS of the authorized duty position, particularly in TOE company–level organizations. Soldiers will only be assigned outside their PMOS or CMF as an exception and with prior approval from HRC. These Soldiers must possess SQI “M” and be the only qualified Soldiers available to fill the vacancy"