Needs clarifikation about adding disks to ASM Diskgroup

Hi,

is it true, that is best practice, to extend ASM diskgroups by adding disks only with same size as the existing disks in it ( ASM diskgroup with different disk size ).

I understand well, that's true within a NORMAL REDUNDANCY diskgroup but we use EXTERNAL REDUNDANCY diskgroups.

Will I maybe get a rebalance issue or so, if  I add a smaller disk?

Regards Peter