So you can use any Atari hardware cartridges providing they can physically fit the area around the slot and the ST(E).
You will probably have noticed there was a MIDEX+ on Ebay the other day. The seller appeared to panic at the price it might fetch and pulled it with a day or less to go. He relisted with a 'Buy It Now' of £65 and it sold instantly. It's strange because there has been another MIDEX+ for sale at only £4 more and that has been hanging around unsold for weeks and weeks.
Footloose and fancy free...gizz a job!