so with a flip flop hub i know most people use both sides go freewheel / fixed

could you go fixed / fixed. or are the fixed fixed hubs something completely different.

could i just thread a cog then lockring on both sides?

There are Fixed/Fixed hubs.

go with a fixed/fixed hub you can thread a freewheel on those if you dont want two cogs but you cant thread a lockring onto a fixed free

The threading for a fixed-gear has two sets of threads - threads for the cog, and threads for the lockring. The threading for a freewheel only has threading for the freewheel (which is typically the same exact threading for a cog).

A flip-flop hub is a hub that is threaded on both sides of the hub. What kind of threading it has depends on the manufacturer, though fixed/free is the most common. Fixed/fixed is available though and is generally preferable as you can thread a freewheel onto fixed threading (Technically you can thread a cog onto freewheel threading, but you won't be able to put a track lockring on to hold it in place).

Can't you thread on a fixed cog on the freewheelside and then thread on a bottom bracket lockring right behind it? I thought they were the same threads as a cog.

There is no point to putting a bottom bracket lockring on after the fixed-cog.

best option = fixed/fixed hub

You can, but the threading on a proper fixed hub is a left hand thread to prevent unthreading. Using the BB lockring has the potential (however low is up to you) to become loose since they are both right hand thread.

i have a fixed fixed hub on my peugeot conversion. its made by maillard