With only 1 PDCH (= 1 air TS for EDGE), you will not need additional ABIS timeslot if you activate flexible Abis. The EDGE Abis timeslot will be taken from the BCCH and SDCCH unused abis nibbles.
So no dedicated Abis TS in your case.
If you need more PDCH, then it might be needed. And you will certainly need much more PDCH than that…? Imagine 2 users sharing one PDCH… etc.