Brian, I agreed with your 1k choice in the sketch, that's what I often use for +4 passive summing. I picked 2.2k for the RCA version only because he said it would be connected to mixer outs. If it was a CD player or something, I'd have said 4.7 or 10k. But without knowing the model number or spec we're both just guessing, I guess.
Twitme, in theory you can passively sum as many signals as you want, but you wind up losing more and more level, and needing more and more make up gain. For 2 signals you lose 6db, like Brian said. Not too bad; you can often just turn up the line trims or raise the faders a bit. For 4 signals it's 12db, more of a problem. With 8 you've got 18db loss, and you start needing a carefully designed amplifier to put it back.
Problems when summing a lot of signals include hum, buzz, and RF, capacitance and instability at the summing point, noise in the gain stage, etc... The more signals you are trying to sum, the more critical these things become. This is one of the big challenges in designing a large console. The DIY approach is possible but tricky. For bigger applications the summing circuit needs to be tailored and fine tuned to the gear around it and it's hard to give useful advice on the internet, because there are so many variables.