starting at the top, and working down
No gravel will make your job easier in a way.
pH of 6.0 is something that nearly all tropicla fish can acclimate to,
however, this should caution you to 2 things: a) acclimate very slowly, and b)
always test the pH of the water you get your fish from to see just how off you may be.
By gradual salt increase I mean take 36-48 hours to build up your concentrations (again to acclimate your fish and reduce the shock/stress).
The recommended concentrations for salt treatment is 2 teaspoons per gallon. 250L is approx. 65 US Gallons, so I would ramp up my concnetrations by pre-mixing (disolve in a bowl of tank water) around 8-9 tsp every 3-4 hours until you got the recommended concentration.
*Also, please remember that you should definetely continue your WCs, just be sure to re-concentrate the lost salt.
Feeding peas is good.
Your Qs:
1) yes
2) yes
3) white stringy-poo is a symptom of parasitic internal infections (meds would be great, but I belive that the salt will work)
3-b) I would only treat with one method at a time and never dose 2 treatments (that's me)
lastly,
please read this:
http://www.aquariacentral.com/forums/showthread.php?t=88601