Typically, any plastic that is "human/food/baby/pet" safe is fine for humans. Ones made with 1 and 2's are the kinds typically found in water bottles, which are fine.
What do you mean, "trimming"? What are you trying to "avoid"?
If youre doing a divider (I assume make your own?) You NEED A SEPERATE TANK! Silicone needs a completley dry tank (if was wet, take at least a day to dry), and then 2-4days to set, depending on the silicon, then a half day to clean once its finished.
Sometimes you need lots, sometimes not too much.
Honestly, I tried doing the DIY divider thing (Im pretty handy) and it just refused to work, which made me maaaaddd. It did it like 5 times, too. I might suggest just getting a premade divider that uses clips. Youre gonna spend at least $5-6 on sealant/silicon (USE AQUARIUM!!!), so you might as well just get the $10 divider from a store!