$500 is the minimum marketable order, i think that's set by the ASX, not by the brokers. but you need to be aware that the "bang for buck" transaction size (where you get the best brokerage % as stated in the schedule eg. 0.11%) is much higher than that, typically around $20K for the CHESS brokers. any smaller and brokerage chews up more and more % of the order size.
the big difference with many online brokers is that they use the custodian/nominee account model ie. when you buy something you are the beneficial owner but the broker is the legal owner of the stock. this means that in the miniscule but not zero chance that the broker collapses, you could potentially lose everything held there. that's a big reason why they're able to offer cheaper brokerage rates as they don't have as many overheads like clearing fees etc., it's more efficient from the broker's POV to lump all customers' holdings together. whereas with CHESS brokers you remain as the legal owner of your holdings, so if the broker collapses those are guaranteed to be safe.
you need to decide for yourself whether you're ok with the risk of losing most or all of your capital held with a nominee account broker in the event that the broker folds, in exchange for the cheaper brokerage rates. the odds of that are negligible, but it has happened before.