Monks gain scimitar proficiency. Additionally, it is classed as a Monk weapon.
Rogues gain scimitar proficiency. They lose longsword and rapier proficiency.
Bards gain scimitar proficiency. They lose longsword and rapier proficiency.
This means that those classes have a choice of a slashing or a piercing finesse weapon. It also means that when a magical scimitar arrives, it is no longer a case of “meh, give it to the Druid.”
Druids get a completely different list. TBA.
Rapier is removed from the game.
Druids can wear padded, leather, studded leather, and hide. They will not wear chain shirt, scale mail, breastplate, half-plate, ring mail, chain mail, splint or plate. These armour restrictions still apply if the Druid is multiclassed.
If a Druid can obtain a suit of normally-metallic armour made of a non-metallic substance (for example, ankheg plate) then they can wear it. These types of armour are available but are rare and expensive.
Sometimes, attempting to use a tool will be impossible without the tools. For example, you cannot work a piece of wood without a blade of some kind. You cannot open a lock without a set of picks.
There is, however, a concept of improvised tools. You can use your dagger to try and work the piece of wood. You can use the pin from a brooch to try and open a lock. Much like an improvised weapon, you will not add your proficiency bonus to such rolls.
Most of the time, you have to be proficient in the tools to use them.
Tool proficiency might apply to knowledge rolls related to the craft. For example, proficiency in woodworking tools may help when identifying dangerous wooden construction.
Note that thief's tools are there to overcome traps and locks. They are of little use when trying to construct traps or locks. You need an artisan's kit for that, and that might not be enough to craft a large item.