Hi there!
First off, could you please explain what you are calling flame opal or where it's from. Not sure what opal this is -- Ethiopian or Mexican? There really is no difference in cabbing opals from anything else you would cab. Obviously you don't want to use your lower grit wheels on them so you don't accidentally grind off color before you realize it. Slow and easy is the name of the game, with a lot of examining your rough before you start working it to make sure you're facing the color bars up to maximize brightness and then during the whole process of cabbing. "Grind a little, look a lot" is the age-old adage for opal cutters.
Shapes are totally up to you. Vince and I cut a lot of non-standard shapes with opal to retain carat weight. However, we will cut off a blemish in a stone and make a smaller, higher quality (and higher priced) opal if the rough warrants it. Non-standard shapes will require custom setting, but that's pretty much the way it goes with opals.
Most opal is sold in jars of water because it shows the color better. Rough always looks better wet. It does not need to be kept in water, however. A certain percentage of opal will crack or craze no matter what -- it is the nature of the beast. It's better to have it crack if it's going to before you sell it to someone. I've worked opals that have been dry for a long time and walked away for a minute, come back to work on the opal and a crack has developed in the cab. Cabbing (and carving) releases stress cracks in opal sometimes and you just can't do anything about it. Sometimes you can cut the crack out or make two smaller cabs, but it's just how it goes. Get use to the fact that it happens. A lot depends on the depth the opal came from and how it was mined.
Solid opals are more valuable than doublets or triplets. The only time you would need to back an opal is if the opal is too thin to set. Sometimes you will come across a piece of rough that is very thin but has outstanding color. You would then make a doublet or triplet out of it to make a cab that could be used in jewelry. A doublet can either be backing and opal or opal and a quartz cap. A triplet is both backing and cap with the opal sandwiched in the middle. A quartz cap will magnify the pattern in the opal and a black backing on crystal opal will sometimes make the color really pop. You assemble doublets and triplets before you cab them. Black backing is usually used for crystal opal. Black potch, obsidian, basenite and black jade are popular. The most popular epoxy to use for making doublets and triplets is Epoxy 330 because it is crystal clear and does not yellow over time. The only time you would consider making a doublet or triplet with opal is if the rough is too thin to cab by itself or the color bar is so thin that you can't risk grinding away the bar. Making a doublet or triplet automatically reduces the value of the opal.
To polish opal you can use diamond or cerium oxide. I have used both. You can use diamond up to 14,000 or 50,000 grit or use diamond to 14,000 and then hit it with some CeOx to make it pop. If you use CeOx, make sure to keep the opal moving and don't let it get too hot. Give it a dunk in some room temperature water if it get's too warm before you continue. You don't need to stablilize opal if it's healthy.
Hope that helps!