At my dojo we practice Jiyu waza which is defending against any grasping attack. This practice usually involves one nage and one uke (occasionally two uke). We also practice Jiju waza which is defending against any attack. Again, this practice involves only one nage and uke. We refer to randori as defense against multiple attackers. As such there is one nage and three or four uke whose job it is to attack the nage with whatever technique they so desire (punches, wrist grabs, lapel grabs, bear hugs). The purpose of the randori is not to have the most technically correct technique but to learn how to deal with more than one attacker at a time. Consequently, many of the techniques are abbreviated slightly. I've found that randori is an excellent way to build your stamina and as you mentioned, your ukemi (you never know what defense the nage is going to use).
"shuchu roku" - Focus all your energy to one point