It's a bit complicated and I'm not sure I understand fully your questions: it's very hard to have this sort of conversations in general terms because the topic is complex. It would be much easier if you detailed what exactly you want to do, like what sort of applications are you have in mind, how those are going to use disk space, networks etc... then we can give you more specific suggestions.
Let me try to add some screenshots to explain some basic things. When you start the wizard to create a new instance you're presented with a lot of options. Let's assume you want to boot from one of the default images. You can choose to a new instance from an ephemeral volume or not using the
Create New Volume button:
No means the new instance will live in an ephemeral volume. Ephemeral means that once you destroy the instance, the volume will also be destroyed.
If you choose to create a volume for the new instance then you get to pick the size of such volume. A minimum size is automatically assigned based on the image you select below. For example, a CentOS-6 image will suggest a volume size of at least 11GB. Ubuntu 16.04 will require at minimum 3GB, etc.
That volume size is the bare minimum to boot the OS: if you think you'll need more disk space for your root and /boot partitions then you need to adjust those sizes.
The 80GB size you see on the
Flavor screen is a default amount of storage included in that flavor.
You can probably resize the volume if the instance is paused or suspended but I wouldn't trust that process. You could attach a separate volume to grow some space but why would you want to do that? so complicated. Much better to destroy and build another instance of the right size. Even if you run WordPress and MySQL on a cloud, you should set things up in a way that you can move the application from one server to another with minimum downtime.
All the storage on DreamCompute is made of the same thing: it's a large Ceph cluster, all SSD. Is this what you mean?
There is a minimum but the optimal depends on what you need to do with it.