The battery manufacturers (Sanyo, Panasonic etc) recommend that all cells in a battery be of the same brand, same type and of the same capacity. Otherwise, all cells will not charge optimally and may not produce their full rated capacity.
The life of your complete investment - airplane, engine, radio, etc. depends on good batteries, it is not a good idea to experiment or gamble with them. Batteries are cheap, airplanes etc are not.