Learn Turkish: İlk evre o kadar kısaydı ki hemen irkildim ve uyandım. - The first phase was so short that I startled immediately and woke up.

The past tense suffix attaches directly to the adjective, right before ki. Since kısa (short) ends in a vowel, it takes the buffer letter -y-, becoming kısaydı (was short). The ki is kept entirely separate as a conjunction, linking the condition to the result.

If I want to say "was so short", can I drop the ki?

Not if you want to connect it to a result! The ki is what creates the "that..." part of the "so [adjective] that..." structure. If you just said İlk evre o kadar kısaydı, it would mean "The first phase was that short," referring back to something you already mentioned, rather than introducing what happened next.

İrkildim translates to "I startled". Does this mean I scared someone, or I got scared?

İrkilmek is an intransitive verb that specifically means to flinch, jerk, or give a physical start yourself. It does not mean to scare someone else. It is the exact right word for that sudden, involuntary physical jolt you get when you wake up abruptly.

Could I use the -ip suffix here and say irkilip uyandım instead of irkildim ve uyandım?

Yes, absolutely! İrkilip uyandım is very natural and smoothly links the sequential actions. The author's choice to use ve instead makes the two actions feel like distinct, equally important events, giving a bit more emphasis to the sudden jolt of startling before the actual waking.