> is it worth building any weaker units to resist specific types of damage
Not really. Offensive scales much faster on the player side than defense. You're generally better off going all-in on DPS than worrying about defense or healing.
> Also, are clea or satana worth a 6* mega RUT?
Both are required at five stars for the Dr. Strange epic quest, but Clea's biometric drop rate is flatly horrid. You'll get at most x3/day. However, she's never been all that hot as a character - even when Dr. Strange was first introduced. Either save the RUTs you may get from the Mutant Chests to put toward Clea or simply farm her while you're working on Hellstorm and Satana.
Satana is a much better character, with a good elemental damage-boosting leadership. Add her Legacy uni & she's an excellent character. You may want to consider a MRUT for her.
Overall, however, I suggest keeping that MRUT for a bit. We'll be getting another game update shortly. One of the new characters may be a better option. If not, Colossus is an excellent contender. You need him at five stars for the Deadpool epic quest - and Colossus is effectively Deadpool's Clea ... except that Colossus is actually useful.