I want to convert an old 12-speed into a 1x6 for urban commuting. I'm hoping to keep one of the existing chain rings at 42T and use washers on the chainring bolts or replace them with single speed bolts. When I remove the front derailleur, do I need to replace it with a chain guide? Do I also need a bashguard? Or is that more for downhill mtb conversions?
I agree with Batman. You can disconnect your front shifter, and use the indexing screws to position the front derailleur to act as a chain guide, and you're good to go.
If you're not interested in doing that, I would recommend paying attention to Emyr's advice--chainrings intended to be used with a shifter have "ramps" on the gear teeth to facilitate shifting. If you're not using a derailleur, these can make your chain drop, which is bad. You might want to get a singlespeed-specific front chainring, they're not expensive.
But, if you keep your derailleur as a chain guide, you'll be fine.
If you're keeping the inner chainring, it's likely to be very similar to a chainring designed to be used as a single ring.
You shouldn't try to use a middle chainring as a single ring because it will have teeth that are designed to drop the chain, something you don't want to happen when climbing hills in the gear of last resort. If the chain drops in this scenario, you will probably injure yourself.
A chain guide should not be required unless you take the bike off-road.
A guard-ring is optional, but may help prevent clothing or shoelaces from being caught by the chain and chainring.