attack priority question regard ranged weapon & magic (in General)

can you equip a ranged weapon and still use a melee DD?
say SoD, and CoC on one minion?

and if not, why not?

Yes.

I am not 100% sure but i think you can if my memory serve me well Warchild had a TOA tank with a ELB + COC.

yes, it was WC.

ToA tank using an ELB with CoC. The CoC fired in melee. This was before the HoC.

from the wiki:

Attack priorities

Melee fighters target your first minion.
Bows/Crossbows/Slings target your first minion, unless you use Mageseeker bow.
Magic Missile targets your last minion.
Enforcer's Crossbow targets your highest str minion. (After all spells take effect.)
Assassin's Crossbow targets your highest Dex minion. (After all spells take effect.)
If you have a Spell/Weapon/Range Weapon, the spell takes priority if or when it is to be cast. I.E. If you have COC/MH/ELB all on a minion, until the COC is able to be cast (Melee Round 1) you will use any equipped items. The first round you can cast your DD spell, it takes priority.
If you have a minion with the skill Phantom Link trained, an amount of damage based on PL's effect that would ordinarily be done to another minion is shifted to the minion with Phantom Link trained. This can override the effects of the Assassin's crossbow and the Enforcer's crossbow.