Hello and welcome to the forum
Judging by the size of the nymph compared to your fingers I would guess L3, and about a month old (3-5 weeks); however, it is a guess as the species and care/food intake amounts is unknown. Well fed nymphs molt much quicker, and some individuals can even outgrow their siblings by a few molts (even if fed the same). The same is true if opposite, underfed nymphs take longer to molt.
There are no obvious physical characteristics on the nymph to be a specific species (may with age/instar though), and with the blurry photos not helping, it is unknown by me at least. Australia has an estimated
160 mantid species too, so it could be just about anything.
Over feeding mantid nymphs isn't really a concern unless your talking massive amounts near daily. I do agree though pet store crickets should be kept in a
simple tank properly fed and watered for 48 hours before use as a feeder. That will make them less likely to injure your nymph, hungry crickets are highly aggressive, and could be loaded with all sorts of nastiness as typically many are forced to eat dead crickets to survive in pet stores.
