I am completely new to list building for seraphon. Need to make a 1500 point army. The models I have are: 20 skinks, 50-60 saurus warriors, one star priest, one bastiladon, 2 carnosaur that can be scar or old blood, about 15 cold one cavalry with 1 scar vet on cold one, 1 astrolith standard, and I think that's all. I have like 5 temple guard as I'm on the middle of building but they won't be done in time
Without a Slann, the Astrolith Bearer under performs. Not Sure I'd include him. Those Knights and Scar-Vet would give you a Firelance Starhost. His CP, plus 2 MW chances per lance can be devastating. I would think about using the Priest as a Starpriest proxy, if you want a dispel. I'd use the Skinks to screen the Priest/Starpriest from deep strike. The Oldblood's CP is also worth using and with a Battalion, you'll start out with 2 CPs. You absolutely want a Bastilidon for it's tankiness. Finally, if you're playing an objective game, use the Warriors to capture and help hold those.
Here's two lists that could work for you: Allegiance: Seraphon Mortal Realm: Ghur Saurus Scar-Veteran on Carnosaur (240) - General - Greatblade - Trait: Thickly Scaled Hide - Artefact: Coronal Shield Skink Starpriest (80) - Artefact: Gryph-feather Charm 5 x Saurus Knights (90) - Lances 5 x Saurus Knights (90) - Lances 5 x Saurus Knights (90) - Lances 10 x Skinks (60) - Meteoric Javelins & Star Bucklers 10 x Skinks (60) - Meteoric Javelins & Star Bucklers 40 x Saurus Warriors (360) - Clubs Bastiladon (280) Firelance Starhost (150) Total: 1500 / 1500 Extra Command Points: 1 Allies: 0 / 200 Wounds: 114 In this list, the Trait and Artefact on the Scar-Vet is for resilience. You want to try and make use of the Firelance battalion as much as you can. Also, I'd suggest running your Warriors and Basti together or near each other. Allegiance: Seraphon Mortal Realm: Ghur Saurus Scar-Veteran on Carnosaur (240) - General - Greatblade - Trait: Disciplined Fury - Artefact: Blade of Realities Skink Starpriest (80) - Artefact: Gryph-feather Charm Saurus Oldblood on Carnosaur (260) 5 x Saurus Knights (90) - Lances 5 x Saurus Knights (90) - Lances 5 x Saurus Knights (90) - Lances 10 x Skinks (60) - Meteoric Javelins & Star Bucklers 10 x Skinks (60) - Meteoric Javelins & Star Bucklers 30 x Saurus Warriors (300) - Clubs Firelance Starhost (150) Total: 1420 / 1500 Extra Command Points: 2 Allies: 0 / 200 Wounds: 108 This list is meant to utilize the Oldblood's CP on the Scar-Vet and give his Greatblad 6 attacks at -2 Rend and 2 Damage. Charge in with Firelance and dish out that extra damage. You sacrifice the Basti, but you can still use the Warriors for objective control or additional screening. This list also allows for 3 starting CPs and a 20 or 30 point Endless Spell.
Those lists are just quickly put together. There are also several great Artefacts that can complement your Carno riders weapons and you'll have the ability to equip two, since you're running a battalion.
Would you say it would be good to get some form of slann to put in there? I'll mainly be fighting stormcast
Absolutely yes. The Slann is the lynchpin of the Seraphon army as its meta is summoning. However, I built those list based on your initial post which did not mention you owning a Slann.