Welcome back Bobo!
1) I'd say if you have enough Daltons then save the Florapol for future use.
2) Daltons when stirred up apparently creates a very murky tank and sometimes results in an algae outbreak. Unless your CO2 and lights are very high, I'd just stick to regular gravel/sand.
3) I've found that if Amazon Swords don't have enough room to grow their leaves get all bent against the glass, not pretty at all.
4) 20L tank? I'd keep it to just the fighter. Pick a really nice one off a breeder, sure beats a sad looking pet store veiltail 8)
5) I would err on the side of caution in this case, I've had fighters jump 5cm+ and having lids gives a piece of mind. Also slows evaporation.
6) RTS and Tiger Barbs can be quite agro, gourami's pelvic feelers/fins may be nipped or nibbled. It's quite a large tank, why not stock a few angels as feature fish along with some bottom dwellers?
7) Yes they are if fed correctly.
-Sam