According to the data sheets:
1) Minimum circuit size supported by the CTX2500 is a VT1.5 (1.5 Mbps). The CE-100T8 also support VT1.5 circuits.
2) All the elements within a Virtual Circuit Group (for a circuit) have to be the same size (n x STS or n x VT1.5). A single card can support multiple circuits, one with STS- and another with VT1.5-based; Table 2 tries to display the supported circuit combinations for an individual card (as there are restrictions).
Hope this helps.