Based on my experience (in part based on what I learned from asking Are small on-flash softboxes useful, or a gimmick? a few years ago), an 8x9" softbox is really going to be too small to be generally useful for portraits. You can use it for headshots, if you get in tight and have it just out of frame, but it's not very versatile otherwise — at least not as the main light source. (As a supplementary tool it can, as the answers to the other questions note, be better than nothing. You can use it for dramatic effect, for example.)
I have a set of Westcott Rapid Box small collapsible softboxes (in both the "strip" and "octa" configurations). These are basically just one size category up from what you're looking at, and I think really the minimum size to consider. If you have the space, I'd go a bit larger.
One of your primary concerns is that your 430 EX II flash doesn't have enough power for this. At the Rapid Box size, I don't think this is a concern; I'm using Cheetah Light V850s, which are cheaper and dumber than that flash but provide about one more stop of nominal power, and I often use them at ¹⁄₄ or ¹⁄₈th power (or even less). And with today's modern sensors, if you need more power than your flash has, you can easily compensate by raising ISO a little bit without meaningful quality loss.
As you can see in my experiments at Does the wide-angle diffuser on a flash help reduce hotspots when used in a small softbox?, my flash has enough power and spread (with a push-on diffuser) to "fill" the softbox. Also, even though they're bigger than the 8×9", you still really will mostly use them just out of frame to get the most benefit, which means that you're close enough that you don't need a lot of power. So basically, I'd cross this off your list of concerns.
You can also go for a shoot-through umbrella, which is an easy way to get a large diffuser cheaply. In my experience, though, they're awkward and harder to use, and basically not worth it overall. (See Will two lights plus octaboxes be a good starter setup for portrait photography, or can I get away with one umbrella? and What is the difference between a softbox and a shoot-through umbrella?) The Westcott products I linked are a little pricey, and you can find generic equivalents easily too. These are nicely made, though, and have an advantage over most softboxes designed for flash because they're made for the flash body to sit outside of the back of the box, making it easier to adjust any settings you need to tweak or to change batteries.
Even then, tweaking stuff at the flash is kinda tedious. You definitely want something that will let you control flash power from the camera. For years I used the built-in wireless flash control of my camera (Pentax's P-TTL), but when I started to get serious about learning how to do studio-style portraits, I found that I really wanted manual control. Automatic TTL just doesn't add much when the situation isn't going to change on its own and when mostly you are concerned with adjusting the ratio between different lights. There are a number of systems that will give you this — I went with the Cheetah Lights I linked above as a low-cost option, or you could choose something like the PocketWizard system, or you could go with Canon's own new radio system. (In a small studio or in a makeshift environment with a lot of light spill, you can get away with optical wireless, and Canon's version of that does allow remote manual power control, so in the interest of budget control and making use of the flash you already have, could start with that and expand to radio later if you decide it'll help.)
That being said, I was thinking I could, perhaps, get away with the Vello FreeWave trigger, attach it to my 430 EX II, and trigger another flash from that output.
– Matt Mar 04 '15 at 02:25