TL431 - negative and positive references point
Can I use those references points at other points in the circuit
without resistor?

You can but, if whatever is receiving the reference voltage can cause it to short out then you need to ensure that the 1.5 kΩ resistor in each circuit is adequately power-rated.

but how do you know what value to choose?

You choose a value that prevents excessive current flow into the TL431 when there is zero load current at the output. So, in your upper picture (the -2.5 volt reference), the TL431 will naturally be taking a current of: -
$$\dfrac{12 \text{ volts} - 2.5 \text{ volts}}{1500 \text{ }\Omega} = 6.333 \text{ mA}$$
And, given that the TL431 can sink up to 100 mA, 6.333 mA doesn't seem much of a problem. I guess for most optimum regulation 10 mA should be aimed for because most specifications in the data sheet assume the TL431 takes 10 mA nominally.
You must also ensure that the current is greater than 1 mA or regulation will be poor.