Taylor and Greg are at least partly immune to Coil's powers.
His Hoist by His Own Petard over Dinah only occurred because Taylor/Ladybird was in the area and got involved after he had already collapsed the timeline. Granted, that could have just been his henchman mockingly asking Dinah the odds of help but it's not the most out-there possibility... Taylor and Greg aren't going to be the only pony form Triggers.
Either Celestia and Luna themselves will change the people's Triggers into pony forms or they will bring in Cadence and Alicorn!Twilight to do the same. This may or may not also involve retriggering people into pony forms, with powers that mirror their original ones. Options, including likely Alicorn interference, include- Canary: Would likely be Cadence (since they both have an affinity with music) though Luna could have some idea of what her original triggering would have caused her and develop a Mama Bear attitude towards the young singer; being in a similar form to Taylor (Canary would likely be a Pegasus, due to both the wings reference and that being Cadence's original form though Unicorn wouldn't be completely out of the question) instead of the dreaded Simurgh would allow Canary a better path in life.
- Theo Anders: Could be any of the Alicorns, due to each of them having their own reasons to Mama Bear him and save him from his terrible father. Again, being in a similar form to Taylor would give Theo a much easier time getting out from under his father's thumb and influence and forging his own path.
- Bitch: Twilight would probably take this one because while Bitch is very broken, she would probably remind the purple Alicorn of Fluttershy and retriggering her into a pony form would likely fix her social issues. After retriggering, Bitch's powers would be more like Fluttershy's abilities, though they may or may not expand to cover animals besides dogs and the "perfectly train them and empower them" powers would still be in place because they're useful. As above, being in a form like Taylor's would help Bitch turn her life around and help the other Undersiders do so as well (because they're her pack and the pack goes where she goes).
- Regent: Could be any of the Alicorns going Mama Bear and his retriggering would probably fix his empathy and the neural damage his father caused him (though this would lead to a Villainous BSoD like in The Warcrafter). His powers would probably change into something a little less villainous but still mind-related and he would almost certainly be a Unicorn.
- Partly confirmed. Canary and Bitch were both transformed, but rather than being done by Cadance or Twilight, they were both changed by Discord, and neither became a pony.
Taylor will become an Alicorn by story's end.
Amy did mention that Taylor's new species has an "ultimate" form they could reach through the proper circumstances and given how timelines generally work out for Taylor...