Here a reply which contradicts to others (mind you, we do this all over the place) :

1.
You can open the tables as you like, as long as they remain free tables. Exclusive or not, doesn’t matter a bit.

2.
No, you can’t change the structure, because the header will get “corrupted” for FP2.6.

3.
You can reindex what you like, however :
We never use Reindex itself though I think I do this from the command window now and then. Anyway, Index On for sure doesn’t give any problems.
Note that any kind of reindexing is not necessary in order to access the tables in both versions.

4.
You can append, replace and delete whatever you like in both environments; nothing conflicts. Locking ? just works the same at both sides.

5.
Yes, one thing for sure you have to do :
Give the table a Codepage of 850 (I think FP2.6 uses 850 always, but depending on the country (localization). When you don’t do this, things like ë, é etc. Will be viewed as other characters. Do this, even when you won’t use this stuff. Think of ° (for degrees Celcius) and mucg more you don’t think of, but your users will. Not today, then tomorrow.
The codepage is in the header, but can get there by the CPZERO.prg which is delivered with VFP, and won’t get the header corrupted for both sides. I.e. you can run CPZERO from FP2.6 as well as from VFP.

6.
Of course you can make any structure-amendment you like, as long as it’s performed in FP2.6.

Good luck, just do it !