[Jeir]

The thing to remember is that pet stores are stores first and foremost. They want to sell things. If they can convince someone that this 30x20cm/11x8in modular cage that costs £50 is perfect for an adult syrian hamster, then all the better; when the hamster starts chewing the bars they can just say 'it's just bored, buy more parts to keep it entertained'. Of course, not all are like that, and there are staff who do want to give the best advice they can, but selling is still the priority.

As for tubes themselves, consider; in the wild, hamsters build huge burrows. Domestic hamsters still have this burrowing instinct. They like tight spaces, because it makes them feel safe (and my own syrian loves cuddles as a result, which is a yay for me). So, if given a tube system, they will try to use it as they would in a burrow, and basically live there, which indeed causes issues with bacteria buildup and airflow issues.

The best kind of cage for all hamsters is one with a large, single floor space. Generally 80x50cm/32x20in or 100x50cm/40x20in is the preferred minimum, although some more nervous hamsters might do better in 70x40cm/28x16in cages (it depends on the hamster!). After that is the matter of depth. You'll want to have the substrate be at least 16cm/6in, as this is just enough for the hamster to start building his/her own burrows.

You can still add plenty of toys to the enclosure, with a wheel (20cm/8in for a dwarf, 28-32cm/11-13in for a syrian), sand pit (which can also be used as a potty), and various hides and cardboard tunnels being favourite. You could even break down the plastic tubes and lay them in the cage if you wished, as it's unlikely a hamster would want to build a house in a tube that doesn't resemble a burrow.

I've rambled a bit, but oh well. Good luck with Oreo. Hopefully he'll enjoy his home once it's all ready for him.