I a small batch of 1-gallon that I'd like to force carbonate rather than prime it in bottles. Is it ok to force carbonate? I realize I may be wasting some CO2, but I'm ok with it. If it is ok, do I need to change the pressure that would carbonate a 5 gallon batch with?
Yes it is ok. But I really recommend filling the keg with statsan, then purge with c02, then add the beer into the out port of the keg, releasing the pressure by opening the safety valve.
If not there's a lot of oxygen in there to oxidize the beer.
Also do not shake to force carb with that much free space, you will foam the whole gallon.