I've fished for brownies and rainbows on stillwater and rivers for many years now and pretty reasonable at casting too.
I've never fished for salmon before and later this year and if things go right I have a few opportunities for a bit salmon fishing on some well known scottish rivers for free (legally) or really cheaply.

Problem is I do not have a dedicated salmon rod, I have a Orvis Helios 906-4 tip flex 6wt fly rod, would this rod be suitable for salmon fishing or would a decent sized salmon possibly wreck it ?

I suspect although it maybe ok the problems might be it would bend too much making landing a salmon somewhat difficult. On the other hand it should be capable of catching a 20lb rainbow off a fishery but I assume the salmon would be a harder fighter.

I would definitely get something with more poke and maybe londer depending on what sort of rivers you are fishing. You are right a 6 weight could probably land a big salmon but that doesn't mean it is appropriate to use one as there are other considerations. The big problems with a 6 weight will be casting big heavy flies that can be are used for salmon, for example copper tubes and similar. You won't be able toturn them over with a 6 weight. Also you will be fishing in a river that will probably be 1 or 2 feet above normal summer flows (i.e. a strong current). Get a big one on a small trout rod in that and you'll have no chance.

For small, tree lined rivers I would suggest a 10 foot, 8 weight. For medium rivers a 12-13 foot double hander with about a 9 weight. For large rivers 14-15 foot plus for a 10 or 11 weight.

When I learned to spey cast I found it tricky at first because I was fairly proficient at single hand casting and the technique is very different and I was trying to use my top hand only. My advice would be get a lesson early on if you take up spey casting...

Thanks for that info, I suspected I was pushing my luck with the trout rod I didn't really take into consideration the bigger flies, tubes etc, what you say makes sense.

I definitely will arrange for a bit tuition on Spey casting.

Thanks