Haha, she admits to a handful. If asked to list her negative traits, however, it'd go something like this. And really... Yuuki is such a honourary CLAMP character it's not even funny.
Short answer: no, no and no.
Slightly longer answer: no, having multiple contracts at a time isn't possible (unless a contract is tailor-made to allow it, I guess) so even if he'd want to he's literally incapable of contracting others while still bound to Ciel. He doesn't want to, though. The thing to remember about Sebastian is that he's freaking stubborn as a mule, willing to play a dress-up game for who knows how many years, nearly kill himself over it on multiple occasions, all to get just one soul - and we know he doesn't need to form contracts to consume souls. It's Ciel or nothing for him.
Now, are there people he'd like to go for once his contract with Ciel is over? Yes. I've mentioned that later seasons!Morgana is like a full bingo card of everything he likes in a soul: noble, brave, willful + years of misery + plunging herself deeper and deeper into darkness, becoming more and more nasty without losing her drive? Yum. The Morgana currently in Kiseki isn't that delicious, but I'm sure he sees the potential.
There are a few others who would maybe do if it were between eating them and starving - Arthur, Elsa, Snow White... They're all too good for his tastes, but they likely would taste okay. Then there are those he thinks it would be fun to work for even if he has no particular interest in their souls. Mainly Jamie, but I think he'd enjoy working with Sherlock too. ...Maybe Art? I'm very canonblind; I'm just going off how Art apparently starts rocking a very villain-y looking outfit for some reason.
Top five most attractive characters in Kiseki according to Yuuki. /deep questions