whew! Sheila, you've described a lot of difficult situations for you and Rufous so I'll make a few general points first....
First - PLEASE DON'T BELIEVE WHAT YOU SEE ON THE TELLY!!!! especially from Cesar Milan who has a less than good reputation in the dog training world, despite the hype.
Back to Rufous:
I think from memory he's an irish boy..? If so he may not have had experience on a lead from a very young age. If he isn't an irish boy, that may still be a factor.
Dogs (and other animals) basically have 2 options if they sense trouble or danger:
i) fight (ie see off the other dog and make it go away)
ii) flight (get the hell outta there yourself!)
on a lead, number ii) is denied the dog - he can't run off so he only has one option - to see off the other dog. If his experience of leads did not start at an early age, then this may be compounded.
Many dogs feel restricted on the lead and are more defensive and less tolerant than if they had been loose. Add to this the possible tension from the owner coming down the lead (and this is very, very common if you realise trouble might be ahead) and you have a volatile situation.
I very strongly suggest you still talk to your vet about possible pain/discomfort. This may make him more likely to react - especially if a rude and silly dog is approaching and he thinks it might jump on him and hurt him.
I once fostered a dog called Fred who had been returned to rescue by two people for being 'aggressive' to humans - this boy was in the last chance saloon, so it was stay with me or be pts. On day 1 when I let him off the lead, I could see he was hobbling - a visit to the vet revealed he had undiagnosed severe HD and a ruptured cruciate ligament. The two families who had adopted him had either tried to drag him off the sofa by the collar (poor love, he had been trying to find a comfy place to rest) or had taken him for long runs that he couldn't cope with since he wasn;t on any painkilling medication. No fault with anyone - they just didn;t realise.
He did get into a scrap with my dog-aggressive rescue - but it was a typical doggy argument over a toy and my own Dexter was the aggressor - he jumped on Fred when they both rasn for the same ball, and fred naturally felt the need (more than other dogs) to protect his weary bones from an idiot lab leaping on his back.
In addition to all this, there is the issue of canine rudeness and the fact that some dogs are not tolerant of in-yer-face social 'loons'. if your dog might have aches and pains, this would be even more likely.
This is quite a long article, but I think you may find it useful to read:
http://www.flyingdogpress.com/sayhi.html
Looking at the examples you gave:
i) The boxer came to say hello. Perhaps Rufous did not want him to. You and the owner chatted, and no-one saw what either dog did - so there's a chance that the boxer was too OTT and upset him. If no-one saw, then maybe Rufous needs the benefit of the doubt - my advice in a future situation was keep an eye on things, learn his body language and look for signs that he might feel uncomfortable - and protect him, if necessary by taking him away or shooing the other dog away.
2. Can you be sure the staffy was messing around after reading the above article? or was it a bit loopy and could he have been worried it might jump all over him (pain/discomfort?)
With the railings - hmm, hard to say but some dogs do react with frustratiuon when there is a barrier between them.
3. Another dog took his toy and wouldn't give it back. Sounds like resource guarding - protecting what's his. if he's a dog who hasn;t had possessions in the past, he may guard them more jealously. This is normal canine behaviour although it doens'yt look nice and as owners it's best to avoid those situations arising (eg when i threw 2 balls and my dog-aggressive dog and the foster boy fred both ran for the same one).
4. Rufous was on-lead? Maybe he didn;t want this big bruiser coming up and sniffing him? maybe it wasn;t polite in its approach, or maybe he sent it polite signals saying 'back off, i'd rather not interact with you' and the dog just kept on coming.
5. Not sure here. One thing worth mentioning is that dogs do not approach other dogs head-on - a friendly approach from a dog is on a curve. However, human life with pavements etc means that we force dogs into situations where they have to walk head-on at each other and then pass close by. perhaps this was a factor - or perhaps there was owner tension - hard to say from the description.
6. Known difficult dog had a go and Rufous responded - he may have felt tense, and Kala may have been uptinght and you too - all adding to the feeling of tension.
He wears a halti harness mostly when we lead walk that attaches both to this chest and to his back so that I have more control of him
I completely understand your need to control him, but this may well make him feel trussed up like a chicken - and even moire likely to explode when feeling threatened, since he would feel so restricted.
I do try to come down hard on him which, may not be the right tact perhaps? I grab him, force him to sit, stand in front of him and tell him firmly 'NO' holding his head either side to calm him down before we go on our way. I am however becoming increasingly angry,
I know how you can end up feeling at your wit's end and terribly frustrated, but sadly you are right that this won't help.
Basically, another dog approaches and he can't cope - so he reacts badly. If you then give him a hard time he will be learning the lesson that other dogs are well and truly BAD NEWS - they even make my owner attack me (in his eyes if you are grabbing and holding his head so he can't move).
being angry when you get home will be too long after the event for him to understand what has upset you (his brain just isn't sophisticated enough to understand) - and if he feels confused, this will just add to his stress and negativity.
As for R's health, he may perhaps be tired at the end of the day and perhaps a bit uncomfortable after 2 previous walks during the day. He sometimes just really doesn't want to go anyway. Perhaps I should leave him and just take Kala?
he may well indeed be telling you something.
It may be that R needs more 1-2-1 time
I would certainly give this a try - but DO check out the medical side of things too.
perhaps if you walk him on his own - fewer walks if needs be - you can feel more relaxed without having to manage Kala as well. If you spot a possible porblem ahead, stay very calm and positive and move him away. praise the good and get in first before anything bad has a chance to happen.
I think I've rambled on enough for now, but this article might also be useful to you:
http://www.flyingdogpress.com/onldagg.html
both are by Suzanne Clothier - she makes very good points, although I must confess that not all her articles do in my opinion if you surf around her website...best stick with these!! :D