Yes, you can mix C7 and C9 twinkle and steady-on bulbs in the same cord if they are standard 20 or 18 AWG socket cord. Check to confirm they are the same style as the bulbs and cords shown in this bulbs and cords category.
This question is closely related to our blog post about installing both glass and LED bulbs in the same Christmas light cord but isn't EXACTLY the same and we see both questions pretty often during the Christmas season.
You can mix bulbs because these 18 AWG cords are wired in parallel so that all the bulbs stay lit if one is completely removed and it doesn't matter if the bulbs have different electrical characteristics.
So,C7/C9 LED twinkle, LED steady-on, Incandescent twinkle and Incandescent steady on, all can be installed in the same cord.
Stylistically, we think that 10-20% twinkle installed with bulbs that are steady on, provides a subtle effect along a roofline. That being said, we have many customers who fill their cords with twinkle bulbs and that's a lovely extravaganza.
Keep in mind the total wattage of the bulbs installed so you don't exceed 80% of the maximum rating of your household breaker (commonly don't exceed 1000 watts per breaker, as a rule of thumb) and never run more than 250 feet of 18 AWG cord at a time in a continuous circuit. This is the maximum light line length of the cord itself and holds true no matter what type of bulbs are installed - or even no bulbs installed for that matter.
Have a specific electrical question? Electricians may seem expensive but we want to encourage you to hire an electrician to take a look at your installation before flipping your switch.
Thanks for all your questions.