Are short ElGamal keysizes acceptable (1024-bit or lower) in cases where security is only necessary in a short timeframe
This got me wondering in general if key sizes are flexible depending on "how long the encryption needs to last" - are there any guidelines here? Any pitfalls I'm not considering?


Yes, you can down-scale the bit-length, e.g. to 1536-bit and it will still likely stay somewhat secure. But do note that for discrete-logarithm over finite fields based cryptosystems there's a steep cutoff in security. Once the parameters are so small that you can break the scheme, e.g. within a week or longer on your hardware, you get "precomputed" data that allows to find any discrete logarithm of interest in a matter of seconds / minutes.

This observation was widely made first with LogJam.