Definitely an interesting build and I'm not sure if what I'd suggest doing is correct.
I would not splash Parcelbeast. You don't get it down on curve and the extra Forest makes your manabase quite a bit worse (which Slitherwisp doesn't like).
You currently have 16 creatures (if I counted correctly); so there's a bit of room to cut down toward 13-14 creatures.
I'd almost certainly cut one Blitz Leech, since the card is quite situational and can get awkward in multiples,
and one Durable Coilbug, since while it ticks quite some important boxes for you, it's also kind of a low-impact card.
If you want to cut another creature, my candidates for that would be Wingspan Mentor or Bushmeat Poacher. The former might surprise you, but I found Mentor to be underperforming quite a bit in general, and your deck already has somewhat big fliers and your smaller creatures generally will take to the skies via Dreamtail Heron mutation.
As for non-creature spells, you have tons of cheap removal, so I'd cut one Dead Weight or Springjaw Trap. The only reason this even is a choice is because Springjaw synergizes with the deck a bit better.
The other card I'd cut is Gust of Wind, you have tons of removal as is, so the bounce effect is probably not where you want to be.
Finally, I'd definitely run the Mutual Destruction from your sideboard. This deck was made for that card; you have enough small stuff to fling at your opponent, it will often have flash so you can choose your removal target to be sacrificed and it can get people out of nowhere (2-mana cast Springjaw + Mutual - they never see it coming
). I'd think Of One Mind would be the cut for that since you already have a decent amount of carddraw, but I could also see cutting one Capture Sphere. You could also consider cutting both of these cards and adding the Dark Bargain.
As for the lands, I think I like running 17 since you have a decent amount of carddraw and the lands mean you candeploy your resources well. You could go down to 16 if you really wanted I guess, but I'd err on the side of 17.
TL;DR:
-1 Blitz Leech
-1 Durable Coilbug
-1 Wingspan Mentor OR -1 Bushmeat Poacher
-1 Dead Weight OR -1 Springjaw Trap
-1 Gust of Wind
-1 Of One Mind OR -1 Capture Sphere
+1 Mutual Destruction
(optional: +1 Dark Bargain over the other one of the last cut options)