will it create an elemental chain if i use ice magic then attack with a attack that does ice damage?

Yes, but only abilities will create elemental chains, not normal attacks. (And if you are using the spell Flare, it will only stack with Fire Units, so Medina using Flare won't stack with Gilgamesh abilities for an ice chain.)

Yes. Even if their attack types are different like magic and slash, ice elemental chain should be made.