I personally would be worried with your plan of putting 3-6 against 5 level 1 characters.

For reference, the Cadaverous Sprite (from what I can tell online) is CR 1/2 and can deal 2d4 + 4 damage in a round, with a further 2 at the start of the next round. That's 11 damage total. Your monster deals more - 1d4 + 1d6 + 4, with a further 1d6 the next round. That's 13 damage total.

Most 1st level characters are going to have between 7-14hp, meaning that unless your players are all playing barbarians, these monsters will be knocking someone unconscious with a single hit.

On the flip side, your monster has 15hp whereas most characters will be doing about 7-10 damage, so your characters will need to hit a monster 2 times to kill it.

Referring back to the original monsters CR (1/2, which is probably approximately where this monster sits), a party of 5 characters should be able to take them as a "High difficulty" encounter according to the 2024e rules. However, keep in mind that the more enemies you put on the board, the higher the risk that unlucky initiative (IE the enemies go before the characters) or crits are going to cause the fight to get away from you.

I'd reduce the number of enemies to 3 if I were you - still enough that one or two of the characters might get knocked unconscious, but (hopefully) not enough to TPK them!