I wouldn’t do a diamond (or any Valenciennes) in a tank less than 4’. They feed on the micro fauna in our sandbeds yet they keep them sparkly clean (Which they shouldn’t be able to do with the lower levels). This is due to them wiping out what they feed on, the nutrients they get from the sand bed may not compare to those that we feed into the tanks. They also rarely accept frozen in the longrun.
Another reason for a 4’ tank minimum is the amount they actually swim. These aren’t like our most common gobies, such as Stonogobiops yasha which usually rest on the sandbed and don’t move much. These do move around and they love to move around more than you’d expect when happy.
So, I do recommend you think about going for a Diamond.
As for the mandarins, you’d need the year old tank (or older). This is also due to them being pod eaters and even when you have them feeding frozen it’s not uncommon for them to revert back to pod eating.