I did a Discus tank a year or so back & it wasn't very succesful at all -
and not for a lack of trying, research or maintenance. To this day I still
don't know why I lost them.

I have purchased a Jewel Vision 260 litre aquarium, and plan on keeping
Discus for the first time.

With Discus the old adage that you don't keep fish , you keep water is more
relevant than ever. What kind of water do you intend to put in it? If R.O.
then fine - but learn how it works & what you need to add to it, if tap
water, you'll need very soft (low Kh) water (& no, not from a water
softener!). If you have hard water you can try peat filtering but you need
to get your head around how it works (this is what I used - and still do use
for my community tanks).

- I plan to keep tropical plants (discus friendly of course)

No problem with that but you'll need plants that work well at higher temps.

I put CO2 on my tank both for the plants and to keep the Ph down for the
Discus. Its a bottled gas system with Ph controller so highly regulated.
Even so I'm not sure its a good way to manage Ph for Discus.

- I would also like to keep some other fish which will co exist in harmony
with the discus. Maybe clownloach, neons and some mollies?

Loaches & neons yes, Mollies, not really - they do best with some salt in
the water which Discuss won't like at all. Avoid things like Gouramis &
Barbs

Where do I start? First of all, how should I cycle the tank? I am not
sure
about a fishless cycle, welcome suggestions - but if someone can recommend
some hardy fish which will be happy in the tank once the discus arrive.

Fishless cyclying just involves getting some pure Ammonia from your local
Pharmacy & adding till you get a sensible level in the water & then
maintaining that while your filters kick in. It probably quicker than normal
cycling. Quickest of all is to kick start the filters with some media from
an existing tank.

Do most community fish such as the species mentioned above adapt well to
the
Discus optimum temperature (It is higher than normal?)?

Discus are best kept at 82-84 AFAIK - most FW tropicals are OK with that.

What is the optimum discus temperature?

82-84

Lastly, I already have an established 100 litre community tank, and I am
happy to do a 40% water change and use the 40 litres of water in the new
tank if it will prove useful. However, I do not want to mess with the
filtration on my smaller tank.

Its not the water you want, its the filter media.

Any help greatly appreciated!

Discus are expensive fish to buy (if you buy half-decent fish). Investigate
whether there are any local breeders that you can talk to visit & then read
up on them - and then read some more. If they aare going to come from an LFS
find out where they get them from & how long they have been bying from that
source. I don't think that there is any doubt that some strains/bloodlines
are much hardier than others.

They need a lot of looking after and lots of water changes i.e. lots of
commitment. You need to think about where the tank is going to go because
they are easily spooked and can damage themselves in the tank. You need to
understand a lot about water parameters & how to maintain them. They are
expensive fish and, having learned the hard way, not something you want to
get into trouble with & have die off on you. Lastly, I'd suggest you get the
tank up & running & established with plants & Tetras etc for 4-6 months
before you even think about putting Discus into it.

rgds

I.