Citric acid is ok. Citric, Malic, Tartaric, Acetic all do the same basic thing in
juice, maybe with slightly different nuances.
At the percentages normally used you won't usually taste a difference.
Read in a
thread somewhere Citric gives a tartier punch & Malic is longer lasting. Dunno bout that but you could experiment yourself & see. Nowadays I use almost exclusively Tart & Sour (Citric & Malic mix) & Vinegar (Acetic) depending on the flavor I'm mixing.
1/4 tsp in 10ml is a good dilution. Bear in mind since you've been using 20% malic this will be a bit weaker (~12%) so you may have to use a bit more to achieve the same effects.
Cheers!