Question
Assalaamu Alaikum

Is it allowed to mix the rates of reciting in a single recital? i.e. Tahqeeq in a few verses, then change to tadweer or hadr in the other verses.

JazaakAllah

Answer
Wa alaikum assalaam wa rahmatullahi wa barakatuh,

If there is a reason for that, then it is not a problem.  For example, if a teacher is demonstrating the different speeds of reading, or a student is practicing them, then there is nothing wrong with changing in the reading. 
If there is no reason for it, then it should be avoided. 
Wa iyyaakum wa-l-muslimeen