Do neurotransmitters compete for space in synapses?

Neurons communicate by presynaptic cells releasing neurotransmitters into synapses, which lets messages be sent to postsynaptic cells. My question is do neurotransmitters compete for space in synapses? For example, Selective Serotonin Reuptake Inhibitors (SSRIs) prevent presynaptic cells from taking back some of the serotonin they released into the synapses. If there is extra serotonin in the synapses, does that interfere with functions of other neurotransmitters such as glutamate or acetylcholine?