That will work. You might find some useful tips here: http://www.tropicalfishkeeping.com/f...quarium-73276/
as it mentions mixing RO with tap/well and the proportions.
On the issue of high pH for corys, I do believe this has an effect on the fish over time. Of equal if not more importance is the probable hardness; GH is not mentioned so I can't say much more on that aspect. The above-linked article goes into how these affect fish. A lot is written about a fish's adaptability, and while some of this is undoubtedly true, one cannot carry it to extremes. The scientific evidence simply does not support it.
While keeping the water conditions good (here meaning clean water, via water changes, plants, not overfeeding, etc) will result in healthy fish, as the store mentioned, this does not mean the fish are actually doing well.
Corydoras panda is now mainly tank-raised, unless you deal with a direct importer from Peru. They manage (or seem to) in water that has more mineral content and a higher pH than the species would ever encounter in its habitat. But it is also the case that these fish rarely live their normal lifespans, and this is most likely due to inappropriate water parameters. Temperature can also play into this, as noted in our profile, even tank raised fish will not last long at temperatures around 79F.
To your (maxw47) original question on the plants, as others noted this depends upon what species, but if the common swords it is likely Echinodorus bleherae or Echinodorus amazonicus [which may be the same species anyway] and these will adapt to higher pH so they may adjust.