1. the lfs charge by lb like most places do thats why you have to take the water out, you dont want to be charged for water, what i do after they are done measuring the weight, i would take some bag water, just to precautious. Yes do not introduce the lfs water to your tank water. I might be silly but i accoumiltate the rock incase of feather dusters and such in the rocks.
2. No they are totally different, scooters arent like any type of blenny, but they are simaliar to mandarin gobies. LawnMower blennies eat algae and scooter eat pods. Scooters are the things that are tricky to feed since they eat pods and are known to wipe out all the podspods in your tank, meaning theres no food for the scooter making it slowly starve to death, ask the local fish store guy to feed it some brine(smaller shrimp) or mysis(bigger shrimp) if it eats it, go ahead and buy the scooter and what the lfs guy fed it. If it doesnt eat it, then dont get it, then you know its hard to feed. You will have to feed your blennie atleast every 2 days from starving, it gets very annoying so if you dont like opening up your hood and waiting for the shrimp to thaw to feed it on a stick or a shooting it from a baster then it isnt your type of fish, if you stick you hand near the scooter it will be scared and run, thats why you need a stick, i use the skewer(the ones in bbqs)
Sorry no info on lawnmowers, heard they are very fun fish to have.
3. You can buy cycle just about anywhere just ask your lfs where it is and they show you. Every lfs that i went to had it. My personal opinion and experience, its not very helpful with cycling the tank, use dead shrimp that will surely cycle your tank quickly then using damsels. I strongly recommend using dead shrimp, sure atleast you'll have fish if you use damsels, but its a pain in the arse to take them out, and if you dont take them out they will attack the new fish since they are VERY aggressive. Dont waste lives, use the dead.
What i would do, get one of those fine mesh bags from the lfs and put the dead shrimp inside and hang it on something and point the powerhead towards the shrimp, not strong enough to move the sand but enough for it to circulate through the bag, once cycle is over all you have to do is throw away the bag, no shrimp pieces to pick up! wish i did that with my friends tank, my back was cramping trying to get the bits and pieces out. Oh there is a strong shrimp odor BUT its only for a week or two, yeah it stinks, after cycling change tank water(25-30%) and the smell should be gone.
Forgot to add in, it will be very cloudy, like a milky type cloud, its normal
Good luck
