Impossible to create neuron with both excitatory and inhibitory connections about carlsim3 HOT 2 OPEN

You're right about that. However, this is not a mere limitation of CARLsim, but is generally true in the brain. It's called Dale's principle, and states that a neuron "performs the same chemical action at all of its synaptic connections to other cells, regardless of the identity of the target cell".

from carlsim3.

Yes, but this means that neuron releases single type of neurotransmitter in all its synapses (actually this is often not true, because neuron can release more than one neurotransmitter), but often single neurotransmitter can have opposing effect on two postsynaptic cells. For example presynaptic neuron could release GABA but one of postsynaptic neurons have inhibitory receptor while the second excitatory.

from carlsim3.