I have recently purchased a .45 RIA 1911A1 and Inherited a .22 Ruger Single Six (And a Remington model 17, that one isn't handgun related though.) from my grandfather. And I am wondering if I have my bases covered between these two?
That pretty much depends on what bases you think need to be covered. There's lots
of niches that can be filled, and different types of handguns fill them more or less well.
If I were stuck with those two, I wouldn't feel much urgency about expanding the collection. Remember, covetousness is idolatry.
or if there is any reason to pick up a 9mm or .357 right away? Other than just because I want them?
You want them is an OK reason.
OK, reasons. The classic two reasons to get a 9mm are, 1)9mm is ubiquitous (everywhere), and 2) it's cheaper.
At the current instant, in round 2 of the Great 0bama Ammo Famine, 9mm can be hard to find. AFAIK, it's still cheaper than other centerfire ammo.
.45 is still expensive.