To meet Iris, you must attend Lily's event, where you buy her a new phone at the electronics shop. If you use cheats to increase her stats, do not bypass the event; you will not be able to get her to spawn as an NPC. As for Christina, you need to be able to go to her house to meet her. You can unlock her house by meeting her other daughter, Emily, and either through Nora's research quests, you can offer to tutor Emily at her house or increase her stats enough to ask her for her address (with the "ask for a favor" option).
As for using cheats, you can modify most NPC's stats and not affect their quests too much but do note that some girls have a "love" and "obedience" path that can lock you into one or the other if you raise both stats too fast. In my experience with the game, the only cheat you shouldn't use is the one that lets you skip research tiers. You can use cheats to increase serum trait mastery (including the quest-related ones like Nora's CBX traits) but make sure you don't cheat by unlocking the next research tier without doing the quest to unlock them (usually by talking to Stephanie). Doing so can lock you out of the research-related quests and upgrades like the nanobots, the Nora research perks (the aunt is a good one to get), and possibly lock you out of getting Ellie.
