C helfeni will get too big, but a little bushier than C balansae(must have 24" or taller tank for this "weed").

I'd opt for something else.

Consider Rotala Green for those sides.
They make nice bunches that can be pruned to look very aesthetic.
Hygros also.

These are not grassy plants, but..........
E vivipara is also an option.

Regards,
Tom Barr