yep, the main thing is that the mattress fits the cot.
I think the SIDS and Kids website has information on the safety recommendations, from memory its no more than 2.5cm gap between the cot sides and the mattress, but check this.

Some cots, my Boori cot being one of them, are a non standard size and so I have to have a boori mattress or it won't fit.

I prefer the innerspring mattresses over the foam ones.