Link budget - Path-Loss in a Wlan Network
Can I just mix dB, dBi and dBm?


Yes you can. For instance, when you said the pathloss was 120 dB but you had a 18.5 dBi antenna, you are OK with then assuming the pathloss is improved to 101.5 dB tempered by another 1.5 dB for cable loss bringing the total to 103 dB path loss.

EDITED section: -

When it comes to 20 dBm EIRP (Effective Isotropic Radiated Power) what it means is that you can't use all the power of 20 dBm in one direction (one lobe) therefore, with an antenna of gain 18.5 dB you can only transmit 1.5 dBm.

In your example, the power at the receiver antenna is 103 dB lower than 20 dBm or -83 dBm (as would be picked up by an isotropic antenna). However, if you used a receive antenna with a gain of 18.5 dBi, the received power will be -64.5 dBm.