If you can go with a rectangle with rounded corners then you should get tempered glass 1/4". It isn't that expensive. I've had two pieces made so far, both were in the $30-40 range.
ALWAYS get tempered glass if you are using glass. It is much stronger than regular glass and regular glass is DANGEROUS if it breaks. Also, if you have exposed edges, you will want to pay the extra amount (about $10 for the pieces I've had, or about 20-25%). If the edges aren't exposed then you don't need to bother (tempering the glass makes the edges not sharp anyway). But the polished edge looks better if exposed.
For your table, you should either go with a rectangle shape or you'll have to go with plexi. Though, normally, I wouldn't recommend plexi as it doesn't look as good and it scratches easily versus tempered glass.
Oh, and both cocktails I have do not have tinted glass. One does have a thinner (1/8?) tinted plexi layer underneath the glass, which I believe is original. It is somewhat scratched so I will need to replace it eventually (already got new glass but some plexi scratches are visible through it).
Wade