What are the dimensions of this 70l tank? For now, I shall jump the gun and presume it is something like 80x30x30cm, while taling about your current fish...
The biggest issue here is getting the true identity of your "Australia Purple Spotted Gudgeon." Is it a Northern veriety (
Mogurnda mogurnda)
http
/www.seriouslyfish.com/profile.php?id=774, a Southern variety (
Mogurnda adspersa)
http/www.seriouslyfish.com/profile.php?genus=Mogurnda&species=adspersa&id=775, or perhaps something else? Both are territorial to a degree, but M. mogurda appears more aggressive and may physically harm your Bronze Corydoras. However, both these gudgeons need a significantly bigger tank because of their adult size (likely to be ~12cm in the hobby), a 120x30cm footprint at least.
Long term, I'd say your Ancistrus temminckii is borderline on suitability for an 80cm. Most ancistrus love strong water current, something that will not go down well with your gudgeon.
I would be very wary of adding new Bronze Corydoras to this tank until you move the gudgeon out, especially if your gudgeon is a good size, they have a huge mouth and could easily be tempted to eat a typical fish store youngster (killing both fish if the catfish manages to release toxin from its pelvic rays upon capture). Without the gudgeon, you could probably up the group to six without issues.
