I currently have both Iron Fist and Luke Cage at LVL 171. The problem I am running into is that Iron Fist was LVL 170 and he tanked 
while Luke Cage tanked 
and 
depended on who I teamed them up with (Often CMags). Problem I have now is that Iron Fist is 171 He tanks
while Luke only tanks
by default leading to Iron Fist with less health dying a lot faster. Any advice on how to keep Iron Fist alive longer?
sell a cover or 2 to let cage catch up in levels. I nearly did that with fist and gamora back when I was playing them more. but then she was featured in an event and she pulled ahead. you could wait to see, but to me selling a cover(s) is worth keeping a team functioning properly.
at the same level, yes. also, there are some areas that there are a couple of levels of leeway where position still works. but it isn’t very intuitive what those are. i know when i was running fist and gamora my fist was a couple levels ahead and i could get gamora on black by position. a third level messed it up, even though the match damage numbers didn’t change. its the unseen decimals and rounding that are generally unknowns.