I've used safe start with great success. However i have not done an instant with shrimp. What i've done in the past is set up a tank, new substrate, plant it well, add safestart. Throw in some of my swordtails or other fish that i have and some snails. Let them go a week max, then introduce shrimp (after removing fish). Its worked for two shrimp tanks ive set up and i've had minimal to no casualties. I guess i've never been brave enough to try it with just shrimp.
My thought process behind what has worked is the fish create a much higher bioload to help with the initial seeding of the bacteria colony. From there the shrimp have a much lower bio-load which for me has worked because the tank was already handling higher levels. And again, heavily planted. Fast growing stems, floaters, and moss. The other step i take is for the first month i feed the shrimp more often than I would on a normal basis on a cycled tank, simply because it takes time for the goodies to grow in enough numbers for the shrimp to have a semi-sustainable natural food source.
This is just my personal experience and i know others have had mixed success. Hope this helps, be sure to let us know what you end up doing and how it works out. Best of luck!