FLR is served with an AVRO, that means bus transfer and limited space for hand luggage. On my last flight to FLR the gate was B8, one of those hidden Schengen bus-only gates at the B concourse. Typically UA arrives at the C gates. Immigration at FRA is not that bad, IIRC the C gates only have a small checkpoint but also less traffic here (and C deplanes early ).
Summary: if there is no significant delay it should work. Connecting from C9 to A 42 is of course a different story but yous still have the sky train to get from C to A.