fusing the four starters obviously seperately, is it worth it or not?

If you are going for every monster then yeah. They are good in PVE but not good for PVP.

i’ve got all the starters already… i’m just scared weather to fuse them or not…

What’s the diffrence between pvp and pve?

Fusing them gives them better stats and more use in the later part of the game, even though they aren’t ideal. You also get a copy of each of the monsters you used to fuse for PVP use only, but it’s always rank E. 

PvP is player vs player, you play other people from around the world. You get to select 15 arks that you have and they are changed from the current level to level 75.

PvE is player vs environment , its the main and side stories where you go through the quests and train your arks.