I believe you can, in theory, reach lv. 5 romance by flirting with any +18 idol as long as they're interested in the player's gender, aren't currently dating and don't have the Asexual trait, though you can only date one idol at a time. I'm not sure how you'd go about lowering romance levels, though.
As for graduating idols: you have the option to convince them to stay a little while longer in the group by dragging them to your office and selecting the appropriate dialogue choice (don't remember which one it is). Depending on how much influence you have over them (and a few other variables such as salary satisfaction, perhaps), they might be willing to postpone their graduation. Otherwise, base idol graduation date depends on how happy they are in the group overall (no depression/injuries, good salary, no bullying, etc.)