30g is the minimum typically recommend for both dwarf octos and cuttles. but that is for only one cuttle, and cuttles do better in small groups. plus they will breed, which helps with the fact that they only live one year. typically dwarf octos last about 2-3 months by the time they make it to the tank.
When they are young then a smaller aquarium is fine, even recommended so you can monitor there feeding. However as adults even dwarf cuttles need a larger tank. Water quantity is important to maintain the proper water quality, plus thy need room to smin and hunt. If cramped cuttles will also fight, often to the death of one or both.
You could live in a volkswagen and probably survive, but wouldnt you be happier in a house...
Cuttlefish are very messy nothing like a reef tank. one inking would be catostophic in a tank that is too small. and yes they would be cramped, and when they are cramped they eat each other. One requires ~30gallons, More than one requires ~55 gallons.
I think your friend's sand burrowers were likely Bobtailed squid. They have a very short natural lifespan (about 8 months I think) and we only see them occasionally on TONMO. I know there is a species from Hawaii but believe there are others as well.
The Octopus mercatoris is the smallest octopus we see. It is a nocturnal species from the Caribbean and has an arm length of 4-5 inches (so an 8 to 10 inch arm span). A 30 gallon aquarim with lots of hiding places makes a nice home for these little guys but the major disappointement is that they are rarely seen since they are both nocturnal and the least active of the animals we keep.
Sepia bandensis is almost the only cuttlefish available in the US (imported, usually as eggs, from the Philippines). This cuttle will grow to about 6" and even if you only keep one, there are issues with them hurting themselves by bumping into the glass if the tank is too small. Additionally, care of hatchlings is very expensive for the first month or two as they need small live prey (mysis shrimp being the proven successul aquarium food). Not only is it hard to keep mysis alive (expect to reorder frequently) but you will need an additional food aquarium form them.
Care for either will be significantly different than keeping a pico reef. We firmly recommend cycling a tank for a minimum of three months, continually building the bacteria, before introducing an animal. Their food will need to be some form of crustacean and just the hidden shells and waste can easily foul the water. You will need to add a skimmer to the set-up (especially important in a smaller tank) to remove ink (in addition to adding to the water quality by removing protein). For an octopus, you will need to design a secure top to prevent unwanted exit. Please visit our Tank Talk section for some ideas on set up and our journals for both care and tank arrangements.
CephKid, You have to watch out for my long winded posts sneaking in there
The cuttlefish is 99% likely to be Sepia bandensis.
See if he can tell you the original body of water for the pygmy. Five years ago this would have been a 99% likelyhood of O. mercatoris but called O. joubini or Caribbean pigmy but we are seeing more variety today and have occasionally seen blue rings (poisonous) shipped when not ordered. If it originates from FL AND it is a dwarf species (identification is an issue with octopuses) then it is still likely to be a merc and a well cycled 35 hex with live rock, secure top and skimmer would do well for this species. If it is an Indonesian animimal, then we are likely talking about a longer armed animal that would not have enough swim room in a 35. I recently kept one that would have been fine in that sized tank but the animals are not identified when caught or shipped and the smaller animals are the oddity, not the norm.