If you want to use HOK, use their products from start to finish. You can go with Auto Air, but the cost difference is negligable if you buy properly.
Personally I would say based on your questions, stay away from the 2 part polyurethanes, buy some dye and some lacquer and some metallic flake. It is far more forgiving, and from you questions I can see that you do not have a lot of experience with spraying these types of products. It is possible to mix between brands, but you don't know what you are going to get, and most times the results are hindered if not a failure. Even if it looks great today, you don't know how it will be in two years.
If you feel you must use 2-part poly's then here is some helpfull info.
Check out Coast Airbrush.
You can buy Kandy Koncentrates. 1 oz. of concentrate is way more than you need. Buy 1 quart of clear and you can mix some up, add the concetrate and now you have a candy color, for only a few dollars more since you need to buy the clear anyway. Coast is nice enough that you can call them and they will sell primer and ko-seal (which you MUST use) in small quantities as a special order. You have to pay a few dollars for the reduced volume, but it is cheaper than buying more than you really need.
Read the HOK manual and follow it to the letter. That will give you all of the info you need as to coat thickness and flash time in between.
As for reducing HOK Clear, you MUST use their reducer. Just follow the instructions. You can over reduce, but allow more flash time and use a slower reducer to prevent solvent pop.