To start with the last question: yes, you can absolutely host more than one service on a single machine, resources permitting. The different services will each listen on a different (TCP) port, and you can front it all with a proxy which works a bit like a front desk, directing the incoming requests to the proper port, so foo.example.com gets directed to service A and bar.example.com gets directed to service B, and so on.
The key part is "resources permitting", because all those services need CPU cycles to run and memory to run in (not to mention storage). Especially RAM is critical; have too much running for the amount available, and your server has to "swap", parking bits of ram to disk, use it for whatever has to run at that moment, and swap bits back. Storage is always vastly slower than memory, so this slows things down tremendously, to the point of the server feeling sluggish or frozen. If you run on a Pi that runs off of a microSD card, not only is your storage really, really slow, you'll also severely limit its lifespan with swapping. So do invest in better storage, like a USB NVMe drive (not a regular USB thumb drive, as those are typically the same flash storage as sd cards). And see if you can get a pi with more RAM. There's no such thing as having "too much RAM".
So, what to run? I don't know about Hubzilla specifically, but their FAQ (under the "average hosting cost" header) says you should be fine — it's just a PHP + database app. But with apps like these, it also depends on the actual use: if your family and friends start following a million people, that's going to increase resource use. Keep in mind that over time, you'll see storage increase slowly but surely, anyway, I'm running a single user GoToSocial instance for myself, and the database and cached images and whatnot amount to some 12GB of storage. I did mention getting extra and faster storage, right? ;) I know there's folks hosting GoToSocial and snac on severely constrained hardware, like raspi zero (so far less powerful than what you have in mind), old routers and even their car radio...
WordPress is just another PHP + database app, although it tends to scale somewhat shitty; if you're not entirely tied to WordPress, you might look into different systems, maybe a static site generator that turns your pages into, well, static HTML files, which take next to no resources (CPU/RAM) to host.
Synapse is a bit of a heavy thing (although it has gotten vastly better, the last couple of years), but it too is quite disk-heavy, so really don't run this on SD cards.
Point is: yes, you can absolutely start with a Pi. I'd try and get one with as much RAM as you can / are willing to spend, as you can't upgrade it, and get some storage that's faster and less prone to failing. But even 2GB will get you some way and you'll learn a ton (aka "break stuff") in the process!
Yep, the M is for mb, that's for RAM in this case.
As for levels... That's not really that black and white, IMO, there's no "best" platform, it's always "it depends". I think you're fine with a pi, certainly for a while, and especially at the price point. Plenty of folks running fedi instances and Matrix off of 'em.
The only thing that comes close and has good software support would be a second hand small form factor office PCs, like HP mini PCs or the Lenovo ThinkCentre. Might buy you some expandability down the road, but it's slightly bigger, uses a bit more energy, choices. It depends.