Yes, that will be fine as a QT.
As for tank size, I agree with Frank...the bigger the better. One thing about keeping lions in "proper" tanks is that if they have the room, they will indeed perch less and will swim around much more than if they feel cramped (or intimidated by aggressive or fast-moving tankmates.
FWIW, we got our P. volitans as a tiny juvie and it has outgrown three setups (6 gal, 24 gal, 60 gal) and is in its fourth, which is a 48" x 24" x 20" 100 gal.