This is a somewhat complicated issue. It is great for you to be thinking about helping others in their development of judo (a particular facet of "mutual welfare and benefit"), but if you are not their regular instructor, then it is not always clear-cut what is going to help them in their short-term or long-term development.
If they are close to you in ability and size, strength, etc., then normally the best thing for them is for you to continue as you have been doing. Doing randori with a better judoka is great for their development all by itself, without you trying to teach them anything specific through it.
On the other hand, if there is a big gap is skill and/or size, strength, etc., then you do need to adapt how your do randori if they are to learn anything other than ukemi and tapping. However, how do you know what is going to help them the most in the short-term and/or long-term? Maybe they do need to improve their ukemi; maybe their movement or posture; their attacking or their defending; their tsurikomi action or their tsukuri; their turnovers or their defending turnovers; their explosive speed or their combinations. You are not going to know what will be most helpful unless you have an ongoing relationship as student and instructor. If I'm doing randori with students I know well, then I'm considering all these possibilities, evaluating and prioritizing what I think they need to work on. If I don't know them well, then I ask their regular instructor what aspects of their judo they should be working on. If their regular instructor isn't around, then I wing it somewhat, and try to work on a variety of things, evaluating how they are doing and trying to help them with their most egregious problems first.
In your example of newaza, sometimes it might be helpful for you to be more defensive and let your partner attack more. Maybe you want to give them some obvious openings and give them a chance of successfully applying the technique. On the other hand, maybe they need to work on their defense more, so you should apply some serious attacks, but give them some openings for escaping or nullifying your technique. You are only going to know what is appropriate if you have some appreciation for their short- and long-term development. If you're not sure, ask their regular instructor, or if that's not possible, try to mix it up at least. Change up how much you attack and defend, vary how big and obvious you make the opportunities for attacking or escaping. But as they get closer to you in their abilities, just aim to be a good tough partner in randori - that will help them a lot in their development.
This post has been edited by icb: 10 November 2011 - 06:36 PM