For example I have a classic sword in the starterPack. When a player joins, the sword is automatically equipped to the first slot ( hotkey 1 to equip ). Is it possible to change it to maybe the second slot?
Unfortunately, Roblox does not allow you to manipulate CoreGui mainly due to security. You could try a custom hotbar instead, or if you really wanted to, you could use a non recommended hacky method of cloning the tool once it’s in the players backpack, then destroying the original one once it has been cloned, thus making the classic sword appear as position 2.
In general though, you could simply just add another tool before the classic sword, which would push the ClassicSword to position 2.