Summon Aberration

It is ambiguous as to whether or not the slaad and mind flayer can fly or is it the beholder only.

They can all fly (30ft), but only the beholder has 'hover', which is a feature that makes flying safer as you will not fall from the sky if knocked prone. 

Since (hover; beholder) applies to fly, that means both should be true. If hover is beholder only, then it should be (hover [beholder]). In 2014 edition, it was fly 30 ft. (hover)(beholder) which indicated that fly was beholder only.

I'd like to get an official "sage advice" ruling on this.

It's worth pointing out that none of the actual Slaad stat blocks have any flying capability. 5.5e Mind Flayers do, but it's very limited and a much shorter range than the spell's stat block.

For that reason alone it seems unlikely that the intent was for the fly speed to apply to all three forms.