Biology problems-spoilers

A couple of things bugged me here.. i) Why sacrifice an entire SJ at the start ? The entire genome of the creature is complete in all of its cells, so a couple of agar plates and some black goo would have done the job. ( this would possibly make sense if the SJs were GM 'tools' themselves and the equivalent of walking petri dishes...) ii) If the SJ at the start is breathing oxygen, then life must already have started on Earth ( atmospheric O2 having been produced by photosynthesis ), I'm assuming the black goo is modifying exisisting DNA on the planet....why should the indigenous life on the planet have genetic material that matches the SJ? ( ..and this would possibly make sense if the SJs were deliberately targeting planets where compatible life had already started...) iii) organisms gain mass by feeding. If something living gets bigger, that matter must have come from somewhere. Squid baby increases in size by a factor of twenty with no available food source. ( this one doesnt make sense at all...) lovely to look at, but as you can see it left a couple of niggles that I think should have been ironed out!