Plants could and should go in asap. They will help with the cycling and actually feed off of the ammonia, nitrite, and nitrates. For shrimp, you'll want a reading of zero for ammonia and nitrite. Once you reach that point, you'll have to do water changes to bring your nitrates below 30. I assume you are aware that you need to be adding ammonia in some form for the cycle to even begin.
Well it doesn't do any good for ammonia to be at 0 if there was never any ammonia present However, the fact you're seeing nitrates means your tank should be cycling. Now it's a matter of waiting for nitrates to go down (though 10-20ppm isn't high, I like them to drop down to 5 before I add livestock, just personal preference).
As far as FSS, not sure if it will support HC but it shouldn't be moving at all when you add water in. Disturbing the substrate is not good as it can release tons of bacteria in the water column...might be non issue now but in a few months with livestock in there could be a big issue. I pour water onto my hand into the tank (hands cleaned thoroughly and dried before I do this). I put my hand right where the back of it touches the top of the water in the tank, then pour the new water into the palm of my hand and let it fan out into the tank. Breaking the flow like that and giving it a surface to spill over keeps your substrate from being disturbed. You could use something like a plate or bowl and hold that if you don't want to use your hand I suppose.
i have fss and it does ok with hc, its kinda light for a substrate so it might take a while for the hc to root well. most people tend to do dry start with hc any way. fss does not leach ammonia and i put my shrimp in with in hours of setting up the tank. DO NOT DO THAT!!!! i had other plants, mosses, dechore and what not that had biofilm in the tank for them and have started many tanks this way. i used rili in this tank for a while with no prob but it does tend to buffer the ph lower than say sand. every one seems to have different results with fss. for some it barley buffers their water for me it takes it down from 7.4 ph to about 6.
1.4 ph drop isn't really 'barely buffering' but I use FSS almost across the board. It is cheap, it is organic, it is attractive. Plain and simple. If you use RO water, substrate isn't as big of an issue anymore.
Read my post again. If nitrates have shown up it is cycling (unless you're using fertilizer which can show nitrates in my experience). And yeah, your LFS guy was wrong...get used to it. I had one tell me Texas Holey Rock was inert...and it's THE aquatic store downtown. You get wrong info, it happens. FSS isn't supposed to leech ammonia. I've got it in two tanks and they never leeched ammonia, I supplied it.