Can Blue Tetra and a Male Guppie Share the same tank?

If the tank is large enough there is no reason why not. The basic rules of fishkeeping are . :- 1 inch of fish needs at least 1 gallon of water. :- Every tank needs a permanently running cycled filter. :- Every tank needs at least 50% of its water changed every week. The fish you have named are both tropical and should be kept in a heated tank at around 75F