In simplest terms oil-in-water has oil droplets suspended in water, water in oil has water droplets in oil. It depends what you are making and which ingredients you plan on using.
LiquaPar is paraben based so it's not as temp sensitive. Obviously you would not want too add it to an ingredient that's boiling.
Optiphen needs to be added below 140 degrees (US). I was told there are no pH restrictions, but it makes my formula curdle, so I don't use it.
Optiphen Plus performs best in formulations below 6.0 pH. The difference between the two is that Optiphen Plus has Sorbic Acid so it's best to have a pH meter for this one.
Phenonip is only oil soluble, so in this respect it works best for oil based scrubs. Since it's paraben based, it's not as temp sensitive. As far as I know there are no pH restrictions.
Your vendor should be able to supply you with a MSDS sheet or you can ask for instructions on usage. Large manufacturers might supply you with the MSDS sheets, but their purchasing requirements are quite large. Quite a few online vendors sell preservatives in smaller quantities. For preservatives, I would try The Chemistry Store or search online. Quite a few soapmaking supply vendors also carry preservatives.