Short answer: no, only Lightning sorcs should self-wield Infinity.
Longer answer: Infinity has two separate relevant mods - Level 12 Conviction Aura When Equipped, and -(45-55)% To Enemy Lightning Resist. The Conviction aura applies to any elemental attacks (including fire) by any party member, but the -enemy res only applies to attacks by the wielder of the weapon. So the -enemy res the only reason to self-wield it, and it only applies to lightning damage. The closest similar weapon for fire damage would be like, a 6x fire facet crystal sword, which works well with the Conviction aura that an Infinity merc supplies, but I don't think sorcs usually sacrifice the FCR to use it. I think some people use them on Fire druids bc those are cooldown skills.