In a flood and drain system, where all plants are being fed from the same water tank, is it possible to grow 2 strains with different finishing times or would the flushing needed for one set of plants comprimise the potential of the other strain? I.E can you put in a strain that finishes at 10 weeks flower and one at 8 weeks into the same system and flush to crop the first without badly effecting the potentcy of the seconed set of plants?

two strains in one system is o.k. but pick your strains wisely... I've got cheese and chronic in the same trays and they get along fine..... when I first grew chronic I got 2 phenos and it was like growing two strains (one finishes at 8weeks, one at 11) one liked more nutes than the other, I reckon you're best off taking clones from one mum and then you can perfect what they like
but two strains in one tray is o.k.

As said above, you can mix two strains that are alike in growth pattern. But to mix different stages of growth would be a mistake because the older plant will take light from the younger one and the nute strength will need to be different for each age of plant.
WG