Isn't having Slafkovsky make Poolparty redundant?

Amen.

How do we get The Firkus Circus if we are keeping Slafcooley?

Not sure what you mean.
Firkus at #37
Sykora at #69
They may both go earlier than that in the draft but I don't think it'd be unreasonable if they were selected around that range.

If it's having space for them on the roster you're worried about, preferably I'd want Slafkooley's ELC to slide into 24-25.
Firkus and Sykora can remain on their ELCs all the way until 27-28.
I'd imagine McLeod, Bastian, Thompson, Boqvist and Marchment all get traded or walks between end of 23-24 and start of 26-27.

If it's the cap, I projected this out all the way through 27-28.
At a conservative estimation of an upper limit at 92.5m, Firkus, Sykora and Slafkooley are all present on a 23 man roster.
Provided the Devils manage to move the last year of Hamilton's contract.

Don’t see why that’d be the case.
Get Puljujärvi in at 22-23.
Let Slafkooley develop until 24-25.

If it were up to me, the only change in the forward lineup from this AGM to the next season would be trading two of Boqvist, Thompson and McLeod out and having (in this case) Slafkovsky drawing in:

Slafkovsky - Hughes - Sharangovich
Holtz - Hischier - Bratt
Gritsyuk - Mercer -Puljujärvi
Foote - Zetterlund - Marchment

Siegenthaler - Hamilton
Hughes - Severson
Mukhammadulin - Bear

Husso
Blackwood

// McLeod, Bastian, Okhotiuk

Firkus will likely go around 10-20. At pick 37 would be a steal.

I’ve been thinking more and more that having Firkus available at #37 is a little optimistic but having him going as early as in the teens?
Idk man, can’t see that happening.
I take your point though, chances of him falling to the Devils are quite low.