It's perfectly viable to start with Downfall characters first if you wish so. But standard characters are probably a little bit less complicated in the very beginning.

Nah start with normal base game first

Start game by normal character is better, these Downfall characters are more complicated then base game characters.

You can absolutely start with the mod, but you'd probably appreciate it more after you become familiar with the bosses and have the normal characters unlocked.

I cant play game with out mod so yes

I would enable the mod right away, but still start with the normal base-game characters. That way the only new content you might see from the mod at first is a few relics or potions, maybe one or two boss cards. Nothing too crazy. When you feel ready, play the Hermit or the Slime Boss.

You should play base so you appreciate what downfall adds and so you can get a hold of the basics before using this mod's more complex mechanics. Idk if this is useful to you anymore cause you posted this 5 months ago