Can my 14 month old wear a t shirt under a long sleeved onsie to sleep in as its cold? Will this create too much tightness on her neck?

Red Nose recommends light weight bedding & clothing that can be added or removed with changes in temperature.
A light weight T- shirt would be suitable as long as it fits well - eg neck not too tight or too loose.

Dress baby and use layers as you would dress or use layers yourself: to be comfortable, neither too hot nor too cold.

Dress baby for sleep and add/remove lightweight blankets to ensure baby’s back or tummy feels comfortably warm to the touch.
Remove hats, bonnets, beanies and hooded clothing from baby’s head as soon as baby is indoors.  - Always sleep baby with head & face uncovered.

A good way to check baby’s temperature is to feel baby’s back or tummy (don’t worry if baby’s hands and feet feel cool - this is normal).
Whether the baby has a cold or infection or another special need.
Consider how many layers that you as the baby’s carer are wearing comfortably.
https://rednose.org.au/article/bedding-amount-recommended-for-safe-sleep