Will Arsenal mags work in Saigas? (.223 specifically)

I don't know about new ones.  But unmodified ones did not.

You will also need bullet guide in the trunnion since unmodified saiga mags have bullet guide but circle 10's do not have built in bullet guide

Other than bullet guide, any other mods needed?

ie: would I need to enlarge magwell for them to fit?

Yes,typically you need to file the front edge a couple times with a file,due to the front trunnion being tall.

Saiga .223 has an AKM style trunion and the SLR 106 has a AK74 style trunion. So to make ((10)) mags fit the Saiga you have take approx. 1 mm off the front of the mag where it makes contact with the front trunion. Beryl 5.56 mags work better in the Saiga although they need the same modification as the ((10)). The ((10)) are awfully tight in the Saiga. Either mag will work reasonably well without a bullet guide with FMJ ammo, though the bullet guide is recommended for reliability.