There are currently 3 brands of "liquid polymer clay" available in the U.S:
1...the one made by Polyform--and sort of the first one manufactured (Translucent Liquid Sculpey)
2...the one now made by Staedtler (Fimo Liquid Decorating Gel or something similar)
3...the one made by Donna Kato (Kato Clear Liquid Medium, I think, called "Kato sauce" in the beginning).
The brands are somewhat different in thickness, clarity, ability to withstand heat, etc, but all will do lots of things.
You can read about them all and how liquid polymer clays are used on this page at my polymer clay "encyclopedia" site for loads of details and even lessons, etc:
http://glassattic.com/polymer/LiquidSculpey.htm
(There's also a thicker version called Kato Polypaste which is great, and tacky, but it's hard to find and out of stock at Donna's store, prairiecraft.com.)
As for your specific questions:
...Liquid polymer clays can be used as glazes but usually aren't in most situations, since to be really clear they have to be applied thin (perhaps in multiple layers), and often heated (briefly) higher than usual which can be a hassle.
(For clear finishes to use on polymer clay--and not use--check out the page at my site called "Finishes"...see below.)
...They are often used when making miniature foods to simulate everything from fairly-clear "jelly" to gravies to frostings, usually tinted/colored.
They can sometimes be used to simulate water, but very-clear water that's deep would be better simulated with epoxy resin or glue, or with polyurethane, or some other things.
...Liquid polymer clays must be cured because they are "raw" plastics. They will be fairly runny until they are baked/heated.
...They make excellent glues, particularly for polymer clay (though may need to be propped/etc for baking since they're not really tacky when a straight liquid).
...(see loads of cool ways to use it on the page linked to above)
...Yes, you can mix solid polymer clays and liquid polymer clays no matter which brands they all are. There are also other colorants you can use when you're wanting colorants, and also "inclusions" like metallic powders and more.
...Check out the page linked to above for Coloring and Inclusions, but there are also pages at my site for Powders, Paints, Inks, Inclusions, etc, with more details.
...I don't know what you mean about using "one instead of the other." If you're just asking again what the differences are the answers will be on the pages I suggested. But yes, you can use the brands interchangeably most of the time as long as you understand they have slightly different properties that might be more useful for some things than others.
Since you're making food miniatures, you might also want to check out these pages at my site:
...Miniatures > Foods
...Houses, Etc > Candies & Sweets
...Clay Guns > Icing Tips
(you can get to any of those pages from the page linked to above using the alphabetical navigation bar on the left side of the page, or you can just use the same nav. bar on the Table of Contents page where you can also see a listing of everything at the site--scroll all the way down:
http://glassattic.com/polymer/contents.htm )
Since you're a beginner, you might also want to check out some of the pages at the site on "the basics" (e.g., Baking, Conditioning, Tools, Glues, Characteristics of Clays, etc), plus the links below at least, which have suggestions for beginners in summary:
http://glassattic.com/polymer/info_letter.htm click on the category called *What All Can Be Done With Polymer Clay Anyway?*
http://www.craftster.org/forum/index.php?topic=73664.msg3086458#msg3086458
easier beginner techniques, explorations
http://www.craftster.org/forum/index.php?topic=192069.msg2020704#msg2020704
beginners tools + supplies + baking
http://www.craftster.org/forum/index.php?topic=73664.msg1267435#msg1267435
HTH,
Diane B.