Got lots of molotov cocktails that I use often, and want to get the Demolitions Expert perk so they do more damage, but is a molotov cocktail a grenade or no?
Yes, in Fallout 4, Molotov cocktails are considered grenades.
From the wikia page on Molotov cocktails, the related perks are listed as:
Demolition Expert, Bloody Mess, SCAV! - Cautionary Crafts
Additionally, Molotov cocktails are listed under the Explosives tab of the Fallout 4 weapons wikia page.
Demo expert will increase molotov damage. Demo expert works with all explosives and a molotov is an explosive.There is some info on a wiki page about it.http://fallout.wikia.com/wiki/Molotov_cocktail_(Fallout_4)