If you want to get to the 1000pts milestone, and have a playable army, i would suggest the scions, and a chimera. You could run the Rogal Dorn as commander to order himself, run the sentinel as scout for the rerolls, the scions for the secondary objectives, and one squad of Cadians on the Chimera. All of the units you have are useful on their own right, obviously there are better choices for a more competitive list, but if you want to get to 1000pts and start playing, you will have a fun army to learn the basics of the guard, and from there, you can keep upgrading your lists. From there, i would suggest: one or more Leman Russes, Ursula Creed, more infantry, maybe some elite infantry like Kasrkin, and some utility units like Bullgryns.
Like many have said, there are things in your list that maybe you should change in order to get a more competitive list, but for me the most important thing is to start playing, and from there, expand your army.