Dean, welcome to the acrylics forum at wet canvas.
Gel medium and modeling paste (which is closer to spackling compound) are two different animals. Gel mediums generally dry clear and flexible while modeling paste (depending on the brand and style) can be less flexible and generally white.
I know of artists that use spackling compound, I am not sure of the ramifications. In short term it may work but the long term issues have yet to be decided.
If your interest is to create archival works, then it may not be the best choice. Sometimes the best choice is to paint within your means. In reality you need
only a limited palette to create beautiful work. There have been several artists that have used a 5 color palette of cyan, magenta, yellow, black, and white. Is it ideal, for some yes.....for some no. Don't overlook the value of a split palette. It uses the basics of red, blue and yellow with a tube of each in warm and cool for a total of 6 tubes of paint plus white and black would be optional.
If you must use modeling paste, there is a paint company in California (I am going to have to research the name and webpage) that has very reasonable pricing on their acrylic products. Don't over look house brand products. Blick, Daniel Smith, and a few others, they have good products at reasonable prices. Always be on the look out for the sale. I know Blick has great sales once or twice a year on their house brand paints.
If you have a chance to go through the information kiosk there is a wealth of information there. There is a link in my signature line.
Found it, the company is Nova Color.