I recently got the Options, which only go down to size US 4 and as high as US11 (I think…) so, I need to round out my KP circs to have sizes for any project etc.
Which sizes/cable lengths are good to round my circs out? thanks!
Which circs to buy?
If you tend to knit in the round on one circular needle, then go for a long cable. Otherwise, it tends to be cumbersome if knitting straight things.
I use US 6 to 11. Anything below that is too small for me, anything bigger too big. It’s a personal choice.
It really depends on what you think you’re going to want to be using them for. KP offers Options fixed circulars in sizes 0 to 3 (with a range of cable lengths) for smaller projects, as well as interchangeable needle tips in sizes 13, 15, and 17 if you were wanting to add the larger tips to your interchangeable set.
I would think about what you have an interest in trying (eg. if you are wanting to try knitting socks, maybe look at the smaller sized fixed circulars or if you are interested in trying out a technique like Magic Loop then look at investing in the longer cable lengths, etc) and then look at what choices are available that would help you to do that.
If you have questions about what might be a good choice for a particular project or technique then definitely ask … there are lots of experienced people here who will be able to offer their advice! 
How often are you going to be using small or really large sizes? I don’t use anything less than a 7, so I haven’t bought a set because there’s just too many sizes I wouldn’t use. It may be better to just buy extra needles as you need them rather than trying to fill in with every single size.
And for when you do need them, the length depends on what you’re going to make. I find a 29-32" size pretty versatile for most knitting; it’s long enough to make a sweater with, can be magic/single looped for small knitting and large enough to handle a blanket. Anything over 36" I find gets in the way, though if you want to knit 2 socks at a time, then you’d need a 40".
I don’t think you need to add anything unless it’s for something you know you’ll make soon or are dying to try. If you really want to make socks with sock weight yarn then yes, you’ll need appropriate needles. If you want to make a super bulky afghan then you may need some larger ones. For the majority of knitting the needles that came with the set are perfect.
I’ve added a few extras as I’ve needed them only and it’s worked fine. I’ve added an extra set or two of the needles I use most (5,6,7,8), extra 24" and 32" cables. I’ve got a few others I’ve added over time, too.
I would like to try knitting socks etc (in the round) sometime, and all the patterns I peeked at need smaller needles than I have?
Okay, so for socks you get the size the pattern says and I think most will be about size 2. Get some and play with them with sock yarn and see how you do on gauge. You may knit loose at first, but then later on as you get used to the thinner yarn and needles will tighten up and be right on gauge. I have a size 2 circ with a flexible cord I can send you. I got them as part of an ebay bid with larger needles which I wanted.
Oh wait, I just looked at them to see what length they are - they’re a 16" 2mm which is a size 0. That may be too small for you and you’d have to do a single loop rather than Magic loop. Anyone else want them?
I’d start with a worsted weight sock which you can do with the needles you have probably. But if you want to use sock yarn I’d suggest a size 1. Tighter makes the sturdier. I learned on dpn, but there are patterns for magic loop and two circs.