Will have IUD put in when cycle starts but need Plan B pill for unprotected sex is that a problem?

It's ok. Taking Plan B before is not a problem. Placing an iud soon after unprotected intercourse is an old fashioned way of proving emergency birth control. As long as you are not already pregnant the iud insertion increases your chance of the emergency birth control working.
Use other method. Since you are planning on an iud, i would recommend another form of primary contraception in the meantime. Abstinence is 100% effective, and may be possible since you indicate iud soon (does not have to be put in at cycle); can use condoms, or regular oc instead of Plan B which is really not a primary form of contraception.
Possibly reschedule. You can definitely use Plan B prior to iud beinb placed, its not dangerous but you only want to place the iud once you know you are not pregnant 100%. You should maybe consider postponing iud placement appointment until you can get tested and make sure you are not pregnant.