I keg ferment all the time. I have 5 going right now.
I put a full 5 gallons in each keg. I don't have issues with blowout and I don't use campden or anything.
The one exception is Hefe. It will blow out krausen. But not any beer. So use a bigger bucket.
Lagers will get a spunding valve immediately on the gas post, set to around 10 psi. No issues with krausen pushing out the valve. after a while It gets pulled and will finish carbing up to target psi.
Ales I will either use a QD and tube on the gas post, into a jar of water, sometimes a little krausen, sometimes nothing. never a clog. once is slows considerably I will just remove the QD and let it carb up at the ending of fermentation.
Ales , I will also remove the gas post and use a 1/2" silicone tube into a jar of water. once it slows I will remove the tube and replace the post and let it carb up as it finishes.
In ALL cases, I have replaced the original "grey" PRV with a "red" PRV. The grey ones pop around 90psi. The red ones are around 25psi. At room/fermentation temps, 25 psi is around the target carbonation point for most beers. So I can basically forget about it and the beer will finish close to the desired carbonation level.
When you chill and tap it, your regulator will either add a little to finish carbing it or you will have a few pints to increase headspace to bleed off any over carbonation.
You can use the "blue" 15psi or "purple" 10psi PRVs if you want even lower psi set points. But it's an "approximate" or "average" psi. they may pop a little higher and reset a little lower.