Yes, pedialyte is OK and good in all such cases, and does some help diarreas in itself.
Pedialyte did big revolution in helping kolera-sick. Almost no dead in kolera nowadays after pedialyte is routine...
If the cat dont manage drink self, use some form of syringe yes or even a straw. Careful, in the corner of the mouth across the mouth, NEVER down the neck.
There are several receipts, the most simple is:
1 soup spoon dextrose-sugar (best) - or honey, in emergency common sugar.
1 tea-spoon kitchen-salt
+ preferably some (a little) bikarbonate.
Everything in 1 litre water; heat up and cool down.
Dont spare too long, this is freshware. Better do make a new portion.
A cat should have as minimum inimum 60 millilitres water per Kg cat weight a day.
Of course more if they have diarrea or lactacing females.
This Pedialyte uou can use at will on both cats and children (did it myself on my son). Not dangerour and usually beneficial.
But that said, do ASAP consult the vet about the diarrhea... It may be something dangerous if your homediets dont help yet.
(I take it you did already tried with diarrea-stopping diets?)