right it depends what substrate you have how you should clean it with your siphon, gravel just push it in and it'll suck everything out, with sand the poo just rests on the surface so you just hover the siphon above it and it sucks it up.
What I do is every week I vac the tanks and change about 20% of the water, I add dechlorinator to the water before adding it to the tank.
You don't need to uproot all your plants to vac, it's bad for them to be disturbed and the poo acts as a fertiliser. Just go very gently and carefully around them. You should however lift up any rocks/decorations etc and vac underneath them.
Weather you should be adding salt depends totally on what fish you have, some of them need it, for some it can kill them so if you haven't already then you'd best check what's right for your fish, if you post your stocking someone will help you.
Bottom cleaning fish such as cory's can be helpful as they eat uneaten food, this stops it decaying and turning to waste, but your always still gonna have fishy poo that needs cleaning.