here's my background. i broke up with my ex whom i have been dating for more than a year a few months ago. it was based on a mutual agreement, so we're at peace with each other.
there's this girl. we took class together last semester. so i met her while i was still dating my ex then.
First of all, don't worry about whether it's rebound or not, just go for it and see what happens.
Personally, I hate playing the texting game, but it sounds like you don't have much choice. As you noted, too much contact will seem clingy. However, you don't want to be completely silent. Send her a friendly message now and then, asking how her summer is going and such bullshit smalltalk. Don't reply too much or too soon. Never say shit like "I wish you were here" unless it she starts saying that kind of thing first. Playing "hard to get" is an old and simple trick, but it works.