Not really an obstacle. Even if it was, they got HARM working on Su-27s in pretty short order. Basic mode, but it got the job done. But there's no need, the USAF has been using Navy missiles for awhile now. I wouldn't worry about it.
As was mentioned, it's all about the radar. We have to hope those F-16s will be packing something better than APG-68s, in addition to HTS pods. They need AESA, that would allow them to get the most of out AMRAAM and HARM both. I've been hoping Russia's penchant for EW and blanket jamming everything would push the powers that be to acquiesce on sending modern radar with those F-16s. AESA has much better resistance to jamming. 90s era APG-68 not so much, it would be useless against cruise missiles and drones in addition to being too anemic to deal with Russian fighters, packing R-77s.
I hope White House and Pentagon just bite the bullet. Focus on how long AESA has been around, give Ukrainian pilots the best chance they can get to finish the mission and return home safely.