I get square stars at SRO (typical seeing around 1.5 arcsec) with an image scale of 2 arcsec/pixel but Drizzle makes them nicely round again
The two times rule is fine for sampling a sine wave in one dimension but not accurate for 2D spatial sampling. Critical sampling is more like 1/3 to 1/4 of the seeing. See for example:
http://www.stanmooreastro.com/pixel_size.htm
Cheers,
Rick.